Protective MBR AFAIK is just a normal part of GPT that is a way to have an MBR while protecting the disk from software “unaware” of GPT that would otherwise treat the drive as if it were pure MBR while it is actually GPT. I have it too on my nvme, 11th gen so I don’t think that is the source of the issue.
I would try the live environment on the T14 and see if the behaviour is the same, if it is detected you might want to wipe/delete all partition then see what happens when you put it back into the Framework and try the live env. there again.
It would be a good idea to inspect the m.2 port for damage or foreign object etc. and assuming nothing looks off try another nvme drive to check for hardware defects with your Framework.
Edit: Also “BIOS” 3.20 is current 11th Gen Intel Core BIOS 3.20 Release and Driver Bundle Update