This sounds like the actual problem. What is the reason an OS will not boot from there?
To me, this does not seem like a minor inconvenience to be worked around–this seems like a showstopping breakage. What if this was your only disk? You would not be able to use the computer at all. I would investigate why booting off the NVME is not working–take a look through your BIOS settings for starters.
Regarding the different Grubs not being able to see each other, the first thing I would suspect is some installations are in legacy mode and some in UEFI mode. Grub can only detect one or the other, so if you have a mix that would explain why the installations on the other drives cannot be detected.
A simple way out of this whole mess would be to use the rEFInd boot manager, which will detect all of the installations no matter legacy or EFI or different disks, Grub/no Grub/Btrfs Grub, whatever. rEFInd - ArchWiki
If you are not interested in going down another path and only want to get a different result from the Ubuntu Grub, it might be helpful to ask in the Ubuntu forums–they might have a better idea why their implementation of Grub is not properly detecting other OS’s.