Okay. Sorry I didn't try this earlier. I am able to reproduce the issue. With Input Quantize enabled, some notes from each take are being dropped on each subsequent loop until I stop recording and play back; playback after stopping plays all notes as expected. And recording without Input Quantize enabled does not exhibit this problem.
Also, it appears that quantizing
is being done on playback of previous takes while looping so I think SONAR must be using something like a live MIDI FX to do this. There are known buffering issues when looping with MIDI FX in place, so I don't think there's going to be a workaround oither than disabling Input Quantize. I never use Input Quantizing so I've not run into this before, but I would be surprised if this problem didn't already exist in X2a.
EDIT: Confirmed the problem it existed in X2a and earlier versions, but didn't seem quite as severe in SONAR 8. It's also sensitive to what the quantize resolution is, and how far off your hits are. Initially, I was deliberately playing way off the beat in order to see what was going on, and have since found that if I set Input Quantize to a high resolution and/or play with more precision that the problem is much less severe or even non-existent. Conversely, some combinations of resolution, pattern and precision can result in almost every hit being dropped.
Time to visit the Problem Reporter.