Hello, we discovered a similar behaviour yesterday when we tried to record a drum loop with an Roland SPD-sx (DrumPads).
What happened is that while we play the record part or are in preview mode everythink works as expected, but as soon as the loop starts some notes just don't play or are shorter than played.
When playing the same track with an e.g. novation impulse, everything sounds like played.
We then tried the same setup in ableton live, and there everything works as expected too.
So we tried to debug the problem with a midi monitor.
What we discovered so far:
The difference between a keyboard and the roland spd-sx is the order of the midi events.
The midi keyboards note on is followed by note off, the roland-spd seams to have the order inverse: first note off then note on. In reality the just arrive very short after another, so that the quantisation puts both event on the same quantisation slice which leads in notes that are on but never get the note off in the loop part of a track.
AFAIK all e-drum machines and pads send the midi note very quickly after each other which will lead to the described problem.
Ableton live handles this that a user can choose if only note on or note off or both should be quantized.
If two midi event on the same note happen within a delta, the note on should always be played before the note off.