@olivierlambert so I booted into Finnix, and was able to mount the root LVM and verify that the filesystem is intact and the files I expect to see are all there. The chroot environment is functional and I don't see anything worry about.
I was able to mount and rebuild the /boot through efi/EFI paths, I have not yet bothered checking file hashes or anything forensic, but I am seeing all the contents in the backups that I see in the running VM from which this backup originated.
One thing I have found, is that only deltas are failing to restore in bootable form. Full backups are fine. I tried to restore the periodic full snapshots within the delta cycles, but those aren't any better than restoring a later delta.
I will be switching to full backups only until I know why delta is broken in my environment.