MIDI Notes Disappearing in PT LE 6.3

[olivix]

I am using Pro Tools LE 6.3. For some reason when I edit notes in one MIDI track, notes in the same positions in other random MIDI tracks occasionally disappear. I don't know what is causing this, but I noticed it happening in one instance specifically when I selected several notes in one track and dragged the ends to change their lengths as a group. It may be happening at other times as well. I am not inadvertanty selecting notes in tracks other than the one that I'm currently working on, and this is not happening to notes in all the MIDI tracks, only to notes in individual tracks seemingly at random.

Is this a known problem with this program? Is that anything that can be done to stop this from happening? Working for 3 hours and then finding out you have several more hours of work ahead of you just to get back to where you were in the first place is not cool.