Okay, this has been driving me
crazeeeeee...not because I get the problem, but because I
don't get the problem. I can't reproduce it, therefore I can't recommend a solution.
What am I not understanding?!?!?
I have a four-bar MIDI loop driving Session Drummer 3 (SD3 is not in playback mode, it's stopped, so the MIDI notes alone are triggering the drum sounds - it's acting like a tone module). I also have a four-bar audio loop. The metronome is turned off for both playback and recording. The four bars are looped.
The loop has been going for about 30 minutes now and I think I'll fall asleep before the two drift out of sync.
The only thing I can think of that's causing your problem is this: Under Preferences > Customization > Snap to Grid, is "Snap to Nearest Zero Audio Crossings" checked (I believe this is the default)? If so, then your audio is almost certainly to be a different length compared to the MIDI sequence, which will snap 100% to the grid as there are no zero crossings.
If the audio loop length is different from the MIDI loop length, they they will drift out of sync IF you used the length of the audio loop to set the loop points because the loop lengths will not be the same.
This is almost certainly not something you will see visually, but being off by even a few samples will cause the two to go out of sync over time.