The three-finger pinch gesture is used in wayland to go to the overview of open windows. That gesture work fine within Okular, too, but unfortunately, it also briefly registers a two-finger pinch. So it does a brief zoom, and unsets your zoom settings, in particular "Fit Width". So it would be great if there was an option to lock "Fit Width" or any other zoom setting.

One could imagine some more fine grained control options regarding when zoom gets actuated, but that might hardly be worth the effort. I have two machines, a surface-style device and a laptop with a touchscreen, and on both the zoom only ever happens by accident. I am sure folks who look at more technical documents with detailed diagram probably appreciate the feature more than me, so I think that behavior should still be there by default.