Sorry to reply again, but I came across this thread from 2022, it appears the same scenario applies, there was even a patch written by @jow (sorry to tag but maybe you might remember this) to help with the with the /boot mounting issue just as I am experiencing
It is as you said, export_bootdevice is the culprit, it seems the patch helped, but it seems to die off and not seeing any followup. I don't have the technical knowhow to implement it, and not sure what the proper avenue is to proceed with.