Hi Anke, I’ve something to add that may help you find the problem.
In brief, the rules of positioning have some edge cases that can affect the behavior. Two things to test and watch out for are z-index changes and 3D transforms, which create new stacking contexts.
If you’re using interactions, they tend to do 3D transforms even for very simple things like moving an element to the right 10px. That stacking context change can e.g. break sticky positioning.
Also I occasionally see mobile-specific differences. I have one site where a slide out interaction is breaking the sticky but only on mobile phones. No CSS change at all, but it behaves differently anyway.