Whilst in some maps, the red dot glass doesn't display. You can't see it in most maps. Riverside and cliffside are some examples. When you can see the red dot glass and you press Q or E to lean from side to side, the glass will follow that tilt and not return until you switch your weapon or maybe even reload.
Not to be confused with this topic: https://superbossgames.helprace.com/i83-red-dot-doesn-t-tilt