So I put my pen onto my graphic tablet from huion for several seconds, with pen mode on, and while ingame in a singleplayer world. This made my head spin very fast while facing down. the tablet is the model H1060P. I also clicked the pen several times to break blocks under myself. This was in creative.

The outcome of this is that I can move my camera up and down freely, but I have to give the cursor a lot more horizontal speed than usual for the camera to begin spinning left/right. In other words, there is now a speed threshold i have to pass with my mouse in order to move my mouse left/right.

Restarting the game fixes this and I did manage to reproduce this. If it doesn't appear to get reproduced, try doing what I did with the tablet for a longer time. Don't forget to move the pen all over the tablet.