This was first brought up on this forum thread:
https://forums.opensuse.org/showthread.php/560223-grub-restore-only-allows-boot-from-1-partition?p=3068979#post3068979

I have 2 partitions of equal size on my laptop, about 26gb each, ext4 format.
They are both used for a root partition, the idea being to have one as a backup
in case one of the tumbleweed updates totally doesn't work.

This is my partition setup:
Device     Start       End   Sectors  SizeType
/dev/nvme0n1p1       2048    1026047    1024000   500M Microsoft basic data
/dev/nvme0n1p2    1026048   55711743   54685696  26.1G Linux filesystem
/dev/nvme0n1p3   55711744  110397439   54685696  26.1G Linux filesystem
/dev/nvme0n1p4  110397440 1943756799 1833359360 874.2G Linux filesystem
/dev/nvme0n1p5 1943756800 1953523711    9766912   4.7G Linux swap

When I run the procedure to restore/install grub, here is what I do. As root in
a terminal:
---
> mkinitrd
> grub2-mkconfig -o /boot/grub2/grub.cfg  
Generating grub configuration file ...
Found theme: /boot/grub2/themes/openSUSE/theme.txt
Found linux image: /boot/vmlinuz-5.14.5-1-default
Found initrd image: /boot/initrd-5.14.5-1-default
Found linux image: /boot/vmlinuz-5.14.2-1-default
Found initrd image: /boot/initrd-5.14.2-1-default
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot
entries.
Found openSUSE Tumbleweed on /dev/nvme0n1p3
Adding boot menu entry for UEFI Firmware Settings ...
done
> grub2-install /dev/nvme0
Installing for x86_64-efi platform.
Installation finished. No error reported.
---

Notice what the result is in the grub configuration file. I will leave out the
advanced options on both partitions because they are the same with regard to
what I found:
---
### BEGIN /etc/grub.d/10_linux ###
menuentry 'openSUSE Tumbleweed'  --class opensuse --class gnu-linux --class gnu
--class os $menuentry_id_option 'gnulinux-simple-ba22e191-xxxxxxxxxxxxxxxxxxx'
{
    load_video
    set gfxpayload=keep
    insmod gzio
    insmod part_gpt
    insmod ext2
    search --no-floppy --fs-uuid --set=root ba22e191-xxxxxxxxxxxxxxxxxxx
    echo    'Loading Linux 5.14.5-1-default ...'
    linuxefi /boot/vmlinuz-5.14.5-1-default
root=UUID=ba22e191-xxxxxxxxxxxxxxxxxxxx  splash=silent mitigations=auto quiet
    echo    'Loading initial ramdisk ...'
    initrdefi    /boot/initrd-5.14.5-1-default
}
---
---
### BEGIN /etc/grub.d/30_os-prober ###
menuentry 'openSUSE Tumbleweed (on /dev/nvme0n1p3)' --class opensuse --class
gnu-linux --class gnu --class os $menuentry_id_option
'osprober-gnulinux-simple-4efad914-xxxxxxxxxxxxxxxxxxxxxxx' {
    insmod part_gpt
    insmod ext2
    search --no-floppy --fs-uuid --set=root 4efad914-xxxxxxxxxxxxxxxxxxxxxxxx
    linuxefi /boot/vmlinuz-5.14.2-1-default
root=UUID=ba22e191-xxxxxxxxxxxxxxxxxxx splash=silent mitigations=auto quiet
    initrdefi /boot/initrd-5.14.2-1-default
}
---

See how on the 2nd menu item in grub for my 2nd root partition, the menuentry
line points to the right UUID for that partition, and the search line also
points to the right UUID for that partition. However, the actual boot line that
begins with linuxefi points back to the other root partition, the same as in
the first menu entry in grub.