It would appear to my untrained eye, that this commit on May 15, 2018 by Ben Hutchings is responsible for this issue:
This appears to be debian specific and not an upstream issue as the upstream code doesn't contain any such checks:
It would appear this is an incorrect assumption to make, that all cards newer than the R600 require firmware. I can't make out what this code is doing without digging way deeper but I think this is what is causing this issue. There would need to be an exception here or the patch would need to be rolled back or came at from a different angle entirely.