It has been a little while since I edited on Avid hardware, but H264 seems to be one of the codecs it didn’t like much. Can you use a different “wrapper” for the file?
About the Sony FX6, in Premiere Pro, there is something weird going on with the codec from that and the FX3 camera. Sometimes it works, and sometimes it doesn’t.
Recently I wanted to transcribe some rushes. Had export audio to Wav file, and insert on time-line to make that happen inside PPro.
Haven’t got to the bottom of it yet why it is that footage from those cameras are not happy.