Happens sometimes in Ground Arcade Battles: when pressing mouse look button ([C] by default), camera switches to a weird angle, focusing on currently selected enemy vehicle and preventing gunner sight from being activated. Selecting different vehicle shifts camera angle, losing sight to the selected vehicle restores regular camera behavior. Pressing C key again makes issue reappear. After the next flyout the issue could no longer be reproduced.
My guess is that it is connected to helicopter flyout.
Demo:
Replay link:
https://warthunder.com/en/tournament/replay/162609527708203363