Usually not a fan of mixdowns but a while ago I was forced to output on a much older Mac Pro with an older version of Avid, and the only way it would export properly is if I did a mixdown. I haven’t seen the problem since but I do know sometimes 6 has a problem exporting from the timeline. Have you tried an export from the bin?
Only other thing I can say is to duplicate the sequence and do a video mixdown.