If you are experiencing this issue can you please try again in the latest Premiere Pro or AME Beta build (22.1.1) and let me know if you are still seeing this problem?
An error occurred while saving the commentneil matthews commented
I reverted to version 14.9 to "fix" the problem (at least to get my work done). Something not right in version 15.0 - typing this from memory, but I believe those version numbers are correct. Unfortunately the footage is not something I would be able to share, and I don't have time right now to make a sample file, but definitely something incompatible with h.264/mp4 when playing in QuickTime player.