We have a small delay already set on all the mixer's feeds in order to get the audio and video in sync, actually. That's exactly what we are doing with a newly installed Soundcraft digital board.
The issue isn't that A/V isn't synced, it's that the
program record that has 16 audio channels (via embedders) does not sync with the
isolated camera records that only have 2 (usable) channels beings they are just aux out of the ATEM and do not run through 5 embedders.
So everything is in sync on all records audio to visual wise... however the iso camera records and the program record can't be synced by timecode in post, because the camera records are getting the AV and t/c signal 5 frames earlier than program is. Similarly, the backup audio record (not as big deal since it's just there for emergencies) which is also t/c jammed, is off 5 frames from the program record for the same reason.
Cheers
W