Per this thread, I ran into the same problem: Delta backups with a UEFI bios won't boot. Instead of adding to that thread, I felt it would be better to start a new thread with the "bug" info in the title. Copying @creoleMalady and @olivierlambert as they were part of the initial thread.

Problem
When Delta backups are taken, the UEFI boot environment variables are not preserved from the full copy. It appears a blank/clean UEFI bios is added to the restore process to get the VM booted. When this happens, the boot list presented to the VM does not include the installed OS. The current workarounds are (1) boot from a rescue disk and reinstall grub, or (2) manually boot the system via the UEFI Shell then add the proper boot disk to the UEFI boot list.

Fix
The fix is to ensure the UEFI boot environment parameters are preserved from the running VM into the Delta copy. This is presumably inside the delta backup code.

Below is the UEFI boot list on a non-working system:

root@Debian-11-delta-restore:~# efibootmgr                                           
BootCurrent: 0003
Timeout: 0 seconds
BootOrder: 0001,0000,0003,0002
Boot0000* UiApp
Boot0001* UEFI Misc Device
Boot0002* UEFI PXEv4 (MAC:0EE793689990)
Boot0003* EFI Internal Shell
                                                    <-- No local boot devices defined.         

Below is the UEFI boot list on a working system (note the last entry "Boot0004* debian"):

root@Debian-11-full-restore:~# efibootmgr                                           
BootCurrent: 0005
Timeout: 0 seconds
BootOrder: 0005,0001,0000,0004,0003
Boot0000* UiApp
Boot0001* UEFI Misc Device
Boot0002* UEFI PXEv4 (MAC:1A5848A7B5DD)
Boot0003* EFI Internal Shell
Boot0004* debian.                                    <-- UEFI boot param kept from full backup 

Workaround 1: Full grub reinstall

• Mount a rescue disk to the VM
• Boot into rescue mode
• Select correct OS disk
• Reinstall grub

Workaround 2: Boot via EFI Shell then re-install grub

• At the UEFI "Shell" prompt, type map to get a list of EFI partitions. On my system, FS0: is the EFI partition
• Type ls FS0:\EFI to get a list of installed OSs. On my system, debian is the name of the installed OS
• Type ls FS0:\EFI\<os> to get a list of grub efi binaries. On my system, grubx64.efi is the bootable efi file
• Run the efi binary. On my system, I used FS0:\EFI\debian\grubx64.efi to get my system running
• Once the OS is running, reinstall grub.

Note: the UEFI Shell expects standard "DOS" partition identifiers. Specifically, note the use of drive:backslash (fs0:\) vs standard unix path/forwardslash (/fs0/)

For additional details on fixing UEFI grub installs, visit this page. It has much more detail on repairing UEFI boot problems.