Mine would possibly not be any good - I only have Manjaro (Gnome, Xfce4, Cinnamon, Plasma …) in several VM’s
not even one on real hardware.
And not in multi boot configuration, but a separate VM for each one.
… and I have altered some of them - to not show a menu or to not wait for any input to select a different kernel …
I trust @Teo to be able to decode the possible issue.
as you can see:
sudo grub-mkconfig -o /boot/grub/grub.cfg
is the equivalent
The package / the command was always available by default in any Manjaro system I installed.
You can install it:
sudo pacman-mirrors -c Germany
or
sudo pacman-mirrors -f
to update the mirrors
sudo pacman -Syu update-grub
(will try to update the whole system and also install that package)