I just faced weird problem. I have a layer controlled by few expressions on itself and on its parents. And since I`ve done one more expression, based on "speed" operator, I can see rotation of bounding box, but the actual image of layer not rotating. Not in viewport nor on render. Just in case - the number in rotation cell also changing during playback.
we need much more information here:
1. full Ae and Os version?
2. show us your setup with screenshots. type EE so we can see what goes on there. try to show a few that illustrate the actual problem
3. since it's a playback problem, if you can show as a video capture