I dont see why this is a problem.
and the main gun is 120mm
there is nothing wrong here

Because tank's main gun is not deisgned to be an AA-cannon.

But it could be a more complicated bug. I noticed that T-100's for some reason attack empty AH-9 after I (playing as BLUFOR unit with setCaptive = true) land and disembark. They don't pay attention to it when I flew around and they don't attack other empty vehicles (cars, apcs) as well. Tested it a couple of times but didn't figure out why T-100's crew hates AH-9 so much (and probably other aircrafts too).

It is certainly possible but it doesn't seem that "authentic" to me, wouldn't a tank would probably try to avoid an attack helicopter unless it was a lucky opportunity or a last resort?

Besides, it has a machine gun on the turret which seems more sensible for attacking something light like a Little Bird.

It's possible IRL to hit a slow flying or hovering helicopter from modern tank's main gun. There were even separate rounds designed for that purpose, although I don't know if they are in use. Shooting at jets is another story, I don't think it should happen.