When you have your gun raised (or ADS) and try to crouch, the game will often lower the gun down for no reason, which desyncs the state with the server (server thinks the gun is still raised up).
This also sometimes happens doing other actions such as leaning. The gun also sometimes instantly lowers itself just after raising it, or doesn't appear to raise at all. But the crouch method is most common and easiest to reproduce, hopefully fixing that fixed the other ones.

This is game breaking and especially ruins PvP encounters.

I have recorded a video on an official server showcasing the issue. There, I entered ADS (single RMB click), then crouched multiple times until the gun lowered. You'll see that I then get desync'd and can't properly move/sprint as the server thinks I'm still ADSing. Then I looked at the sky and clicked LMB multiple times to "shoot". After ejecting the magazine you can see that it is now empty.
https://youtu.be/xFTFhZB7dO4

I believe this started occuring very frequently since 1.18. Please put this on highest priority, as it has tremendous negative impact on PvP.