UEFI boot issue: Replaced M.2 and returned it back to the system, now Q4.2 unable to boot (in between installed Q4.1 on the replacement)

[apparatus]

This happened because you’ve removed the disk with installed Qubes OS and booted the PC.
On boot the UEFI checks all entries and remove the ones that point to non-existing disks.
You can recreate the UEFI entry like this:

BIOS Update That Will Break Boot

If you use uefi : get a qubes os media installation. boot to qubes os rescue mode. skip to shell. [shell] : efibootmgr -v -c -u -L "Qubes OS" -l /EFI/qubes/grubx64.efi -d /dev/nvme0n1 -p 1 reboot change -d <which drive?> and -p <which efi partition?>. if you want you can make sure first by issuing efibootmgr -v in dom0

And it’ll work until you remove the disk with installed Qubes OS and boot the PC again.

The UEFI is using fallback path by default if it exists.