It happened to me.
Updated bios and suddenly manjaro /boot is not found, only windows boot partition.
I have reverted back to the old bios firmware but it did not help.
I have backups (well, I have 2 and one is from right before the bios update so that is probably worthless) of the partition (and full disk for that matter) I can try, but I don’t think that will change anything for me since nothing was changed on the drive, it worked right before!
I chrooted into my installation mounted the boot and ran an update/grub and it worked perfectly, so it doesn’t seem to be any problems with the boot partition or filesystem.
Still marked with boot flag in partition manager.
This is the output from the live session I am in right now. sda5 is my manjaro boot.
NAME FSTYPE FSVER LABEL UUID FSAVAIL FSUSE% MOUNTPOINTS
loop0 squash 4.0 0 100% /run/miso/sfs/livefs
loop1 squash 4.0 0 100% /run/miso/sfs/mhwdfs
loop2 squash 4.0 0 100% /run/miso/sfs/desktopfs
loop3 squash 4.0 0 100% /run/miso/sfs/rootfs
sda
├─sda1 vfat FAT32 WIN-BOOT 5483-0EFB
├─sda2
├─sda3 ntfs windows 6654845D5484323B
├─sda4 ntfs windows-extra E61A49731A4941B3
├─sda5 vfat FAT32 AC73-C072
├─sda6 swap 1 5ae8fbf5-a704-41ea-be6d-bf26b38325ed
├─sda7 btrfs 700d034d-b99a-482b-880b-9060acf3a6b3
└─sda8 btrfs 2a875645-5701-4450-bd08-adf1f966a07c
sdb
├─sdb1
├─sdb2 ntfs Lilla Fil-fitt disken
│ CE8C0EC38C0EA655
└─sdb3 ext4 1.0 MISO bd6e3634-1f2f-405f-87a1-5115bda000de
sdc
├─sdc1 exfat 1.0 Ventoy 19E1-C0B4
│ └─ventoy
│ iso966 Jolie MANJARO_KDE_2210 2023-04-21-10-04-39-00 0 100% /run/miso/bootmnt
└─sdc2 vfat FAT16 VTOYEFI 1E1C-88C8
nvme0n1
├─nvme0n1p1
│
├─nvme0n1p2
│ ntfs Games 68A4C531A4C50294
├─nvme0n1p3
│ ext4 1.0 backup 2b1e42bb-fe37-4fa1-991e-aab694b6b16f
└─nvme0n1p4
ext4 1.0 Games ccfef3ee-65e0-45e4-aac2-4604ad2e696d
What the heck do I do?!?
edit
Just to make clear I am using efi not anything else.
sudo fdisk -l /dev/sda ✔
Disk /dev/sda: 465.76 GiB, 500107862016 bytes, 976773168 sectors
Disk model: Samsung SSD 860
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: AA9D4D47-282A-4A7F-AAB9-C46C81E52DDC
Device Start End Sectors Size Type
/dev/sda1 2048 206847 204800 100M EFI System
/dev/sda2 206848 239615 32768 16M Microsoft reserved
/dev/sda3 239616 419670015 419430400 200G Microsoft basic data
/dev/sda4 419670016 420966399 1296384 633M Windows recovery environment
/dev/sda5 420966400 422014975 1048576 512M EFI System
/dev/sda6 422014976 501706751 79691776 38G Linux swap
/dev/sda7 501706752 757559295 255852544 122G Linux filesystem
/dev/sda8 757559296 976773134 219213839 104.5G Linux filesystem