is the internal camera audio in sync and just the h6 out of sync? that may tell you something. it aint’ no f8 and most people report that it drifts after 3 minutes of running.
also, 24fps may actually be 23.976 fps and 30fps may be 29.97. and 60fps may be 59.94.
also, i’ve read that the timebase being set wrong can do weird things.