Do you have more than 120 notes on that track after copying? Maybe that’s the issue, as that’s the hard limit on the XY. 120 notes per track, per pattern.
You were right about the limit. I created a new project so as to not mess up mine, and created a busy drum loop, and sure enough it wouldn’t fully copy to bar 4 until I made it less busy.
Thanks for your help and also to TE support for getting back to me