1. Increase Collective is Shift. I don't know why you mention Q.
2. Moving the mouse changes the attitude of the rotors. It changes the orientation of the rotor plane, not individual rotor's angle i.e. not collective.
3. I'm talking ArmA3. Are you?

Of the many, many hours I've flown helo in A2, I have never ever seen this behaviour.
Of the many times I've gotten into helo in A3 (I use that situation as a rough benchmark while adjusting video, and for testing new OC) I have only twice (2 times) seen this happen.

I tried a number of times to make this happen again. It never did, except for this second time, quite by accident.

So you're saying that twice the helo responded properly, and all other times (where it required actual keyboard input to Increase Collective) it did not? That what I took as very acceptable and predictable behaviour was actually a failure mode?

FWIW I have never heard that mouse movement had any effect on Collective setting. Certainly not while motionless on the ground.

w/respect, I think you have a notion. but nothing more.

Lifting off when the mouse is moved back and forth ... that's "as they should"?

If you mean that they usually do, yes, you're right, that's true.

Didn't get a change to do a lot of test, but:
Deleted Shift and Z.
Put Shift and Z in Increase/Decrease Analog.
Much more responsive. BUT: needed constant / often Increase to maintain alt, and AutoHover didn't seem to maintain either.
SO: I put Shift back into the original position (Yes, so it appeared in both.) and ... definite improvement.
Again: didn't spend more than a few minutes, but def better descent, and (I think) also rise.