Update: I tried flying a little bird and at times, w would fail to lift the chopper. But if I pressed spacebar, w would start working again. So I rebound the jump binding, which is also the MAV raise key to b. Worked like a charm. The MAV now has no problems unless I press spacebar. Editing rather posting so as to not bump what seems to be a personal problem.
Edit: Map seems to have no effect, it actual happens everywhere so far as I've seen recently. In addition to w making you go up, a makes you go left and backwards, and d makes you go right and forwards.
On Seine, Pipeline and Norshar, the MAV seems to fly fine. But on Metro and Karkand, whenever you press w to go forward, it also goes upwards.
Here's a video of Karkand with on-screen keyboard:
https://www.youtube.com/watch?v=iODQIM5RlYw
Notice near the middle I could press s to go backwards and this would temporarily make w function act properly.(Which makes me think it's not a personal/binding issue) It could have something to do with conquest as the Metro and Karkand instances were both of conquest whereas my use of Seine, Pipeline, and Norshar were of Rush/TDM.
It's not easy to test with map rotation/other players.