hi, care to elaborate or give some sure steps to reproduce the issue you just found ?
it might be very and/extremely specific to some settings (eg. tempo/bpm, time-sig, snap-to-beat, etc.) and to any prior clip-edit operations you take, like cut, move, resize, whatever applying to the precise first note onsets just before the very beginning of the clip
notes don't just disappear from clips--they're probably still there in the underlying .mid file for which the clip is just a "window"-- also, events that start before the clip-start location are not part of the clip (or not seen through the window)
please clarify what's the issue.