After you have updated the system to latest mkinitcpio 28-1.0 you can fix your system with:
$ sudo mv /boot/vmlinuz-linux54 /boot/vmlinuz-5.4-x86_64
$ sudo mv /boot/vmlinuz-linux58 /boot/vmlinuz-5.8-x86_64
$ sudo mkinitcpio -P && sudo update-grub
Thanks a lot ! I’ll try that right away.
Edit : @philm worked like a charm. Thanks again for the quick fix.
Held off from updating 'til the fixes were in place, and everything seems to be in order post update.
I hadn’t seen the specificity of this update.
And I unfortunately reboot.
At the grub edition, I get this:
Setparams 'Manjaro Linux'
savedefault
load_video
set gfxpayload=keep
insmod gzio
insmod part_gpt
insmod ext2
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root 7aab3ee3-4bba-4f13-8772-2a24627f5be5
else
search --no-floppy --fs-uuid --set=root 7aab3ee3-4bba-4f13-8772-2a24627f5be5
fi
linux /boot/vmlinuz-linux58 root=/dev/nvme0n1p2 rw quiet apparmor=1 security=apparmor udev.log_priority=3 initrd /boot/intel-ucode.img
Now I don’t know what to do, I don’t have access to the tty anymore, etc.???
reboot with usb iso manjaro
you will do a chroot for repair
After reading announcement, I unchecked kernel-updates.
No errors. -> Reboot. Installed kernel-updates. -> Reboot:
everything o.K.
OK, thanks. Work fine 
no apparent issues here 
(5.8/AMD/KDE)
I still got error on my second monitor with KDE Plasma, Linux 58, on every boot. I have fix with reconfigure on settings-> Display, but still happened after reboot.
This is an issue since
I do not plug in/out, just open settings => display, then reconfigure on dual monitor setup, like drag second monitor the right, until show [Apply]. It fix it, until reboot.
no this problem on my Testing Dell Precision 4800 and on USB Unstable, both Plasma
Rebuilding certificate stores…
Updating module dependencies…
Updating linux initcpios…
==> Building image from preset: /etc/mkinitcpio.d/linux54.preset: ‘default’
→ -k /boot/vmlinuz-5.4-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.4-x86_64.img
==> Starting build: 5.4.67-1-MANJARO
→ Running build hook: [base]
→ Running build hook: [udev]
→ Running build hook: [autodetect]
→ Running build hook: [modconf]
→ Running build hook: [block]
→ Running build hook: [keyboard]
→ Running build hook: [keymap]
→ Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.4-x86_64.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux54.preset: ‘fallback’
→ -k /boot/vmlinuz-5.4-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.4-x86_64-fallback.img -S autodetect
==> Starting build: 5.4.67-1-MANJARO
→ Running build hook: [base]
→ Running build hook: [udev]
→ Running build hook: [modconf]
→ Running build hook: [block]
→ Running build hook: [keyboard]
→ Running build hook: [keymap]
→ Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.4-x86_64-fallback.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux57.preset: ‘default’
→ -k /boot/vmlinuz-5.7-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.7-x86_64.img
==> ERROR: ‘/lib/modules/5.7.19-2-MANJARO’ is not a valid kernel module directory
Hiba a(z) konfigurálásakor
==> Building image from preset: /etc/mkinitcpio.d/linux57.preset: ‘fallback’
→ -k /boot/vmlinuz-5.7-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.7-x86_64-fallback.img -S autodetect
==> ERROR: ‘/lib/modules/5.7.19-2-MANJARO’ is not a valid kernel module directory
Then I ran mkinitcpio generation, hope it will result a bootable system. 
sudo mkinitcpio -P && sudo update-grub
==> Building image from preset: /etc/mkinitcpio.d/linux54.preset: 'default'
-> -k /boot/vmlinuz-5.4-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.4-x86_64.img
==> Starting build: 5.4.67-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [autodetect]
-> Running build hook: [modconf]
-> Running build hook: [block]
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.4-x86_64.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux54.preset: 'fallback'
-> -k /boot/vmlinuz-5.4-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.4-x86_64-fallback.img -S autodetect
==> Starting build: 5.4.67-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [modconf]
-> Running build hook: [block]
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.4-x86_64-fallback.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux57.preset: 'default'
-> -k /boot/vmlinuz-5.7-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.7-x86_64.img
==> ERROR: '/lib/modules/5.7.19-2-MANJARO' is not a valid kernel module directory
==> Building image from preset: /etc/mkinitcpio.d/linux57.preset: 'fallback'
-> -k /boot/vmlinuz-5.7-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.7-x86_64-fallback.img -S autodetect
==> ERROR: '/lib/modules/5.7.19-2-MANJARO' is not a valid kernel module directory
==> Building image from preset: /etc/mkinitcpio.d/linux58.preset: 'default'
-> -k /boot/vmlinuz-5.8-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.8-x86_64.img
==> Starting build: 5.8.11-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [autodetect]
-> Running build hook: [modconf]
-> Running build hook: [block]
==> WARNING: Possibly missing firmware for module: xhci_pci
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.8-x86_64.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux58.preset: 'fallback'
-> -k /boot/vmlinuz-5.8-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.8-x86_64-fallback.img -S autodetect
==> Starting build: 5.8.11-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [modconf]
-> Running build hook: [block]
==> WARNING: Possibly missing firmware for module: xhci_pci
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.8-x86_64-fallback.img
==> Image generation successful
Yepp, it is bootable. Intresting, my other PC have no problem with this update.
this because in /etc/mkinitcpio.d/linux57.preset file is still present , but no more linux57 , i have suppress preset for version linux no more installed
Just updated on a Dell XPS 13; (5.8/Intel/Gnome). No problems at all! Thanks for fixing this so fast Phil!
you can create a script in ~/.config/autostart-scripts to restart the plasmashell after boot automatically.
All fine on XFCE + BTRFS + linux58 only the mkinitcpio -P failed on the Linux 5.7 Kernel not necessary to fix, because this Kernel has EOL state.
only the extra modules exist for 5.7:
root:~/ # ls /lib/modules [20:29:58]
5.4.67-1-MANJARO 5.8.11-1-MANJARO extramodules-5.4-MANJARO extramodules-5.7-MANJARO extramodules-5.8-MANJARO
Just a warning during the update process:
(64/90) mise à jour de manjaro-zsh-config [----------------------------------------------------------] 100%
avertissement : les permissions pour le répertoire /root/ sont différentes
système de fichiers : 750, paquet : 755
It warns that /root/ folder has different permission (750) when manjaro-zsh-config has 755
Not sure if it is my folder which has weird permission (I doubt it 750 seems about right for /root/) or the packet itself which has non standard permission.
got the same warning.
i think 750 is appropriate for /root/. though seen on many other distros having ability to peek in /root/ with normal user, could only be possible with 755 i guess