I am using split MIDI/audio tracks.
 

Thanks for your reply Ron. On any of the split MIDI/audio tracks, do any of the synth audio tracks have the Gain knob adjusted to something other than 0. If so, does setting that/those gain knob/s to 0 db fix stop the sync problem for you?

I have no idea but could this be due to a clock issue?

Will try and get back to you.
 Thanks Ron.

Setting the gain knobs to 0  does fix the sync problem.
 Thanks for your help Daniel Cate.

What is it that you are calling split MIDI/audio tracks?
 
I have an instrument folder with the midi and separate sudio tracks. I also have the project Craig worked with if you want to play with it. It will need some audio trimming and bounceing of the clips before groove clipping. I can OM you the download location if you want.
 
btw, some of us don't monitor this forum all day.  ;) I just came in from mowing the lawns.

Dan Cate [Cakewalk]
Thanks for your reply Ron. On any of the split MIDI/audio tracks, do any of the synth audio tracks have the Gain knob adjusted to something other than 0. If so, does setting that/those gain knob/s to 0 db fix stop the sync problem for you?

Whoa. Good one, Dan. I can reproduce the problem with non-zero gain on the synth output.

Thanks for your help in tracking this down. Since 8.5 there have been about 3 bugs that exhibited the same symptoms of MIDI drift during looping. 2 were fixed during the X1 and X2 cycles. Hopefully this will be the last one. In the meantime, keep the Synth audio track gain set to 0 and use the synth's output for gain staging before the track fader or use metronome playback to avoid the issue.
 
If anyone is experiencing MIDI sync issues during looping that still occurs with setting the Synth's audio track gain knob to 0, please let me know.  

Thanks for your help in solving my problem.
I love X3 .

Dan Cate [Cakewalk]
Thanks for your help in tracking this down. Since 8.5 there have been about 3 bugs that exhibited the same symptoms of MIDI drift during looping. 2 were fixed during the X1 and X2 cycles. Hopefully this will be the last one. In the meantime, keep the Synth audio track gain set to 0 and use the synth's output for gain staging before the track fader or use metronome playback to avoid the issue.
 
If anyone is experiencing MIDI sync issues during looping that still occurs with setting the Synth's audio track gain knob to 0, please let me know.  

Dan, can you please comment on my post above?