I have no idea what the problem was that grub-probe failed, but it looks like
this error is somewhere ignored. No idea if by grub2-mkconfig or by
transactional-update. In the end this leads to an unbootable system, as grub2
is completely broken and doesn't even offer a rollback option anymore:

microos:~ # transactional-update grub.cfg
Checking for newer version.
transactional-update 4.7.0 started
Options: grub.cfg
Separate /var detected.
2024-08-23 13:23:55 tukit 4.7.0 started
2024-08-23 13:23:55 Options: -c8 open 
2024-08-23 13:23:56 Using snapshot 8 as base for new snapshot 9.
2024-08-23 13:23:56 /var/lib/overlay/8/etc
2024-08-23 13:23:56 Syncing /etc of previous snapshot 7 as base into new
snapshot "/.snapshots/9/snapshot"
2024-08-23 13:23:56 SELinux is enabled.
ID: 9
2024-08-23 13:23:58 Transaction completed.
Creating a new grub2 config
2024-08-23 13:23:58 tukit 4.7.0 started
2024-08-23 13:23:58 Options: call 9 bash -c /usr/sbin/grub2-mkconfig >
/boot/grub2/grub.cfg 
2024-08-23 13:24:00 Executing `bash -c /usr/sbin/grub2-mkconfig >
/boot/grub2/grub.cfg`:
/usr/sbin/grub2-probe: error: failed to get canonical path of
`/proc/self/fd/6'.
2024-08-23 13:24:00 Application returned with exit status 1.
2024-08-23 13:24:00 tukit 4.7.0 started
2024-08-23 13:24:00 Options: close 9 
2024-08-23 13:24:02 New default snapshot is #9 (/.snapshots/9/snapshot).
2024-08-23 13:24:02 Transaction completed.

Please reboot your machine to activate the changes and avoid data loss.
New default snapshot is #9 (/.snapshots/9/snapshot).
transactional-update finished