Given the rviz Pose display transforms pose data into the fixed frame (see here), there might be something going wrong there. Do you get a error message in the Display output on the left? There might be a timestamp problem that prevents conversion of the pose to the fixed frame.

I think I found the problem (at least the problem hasn't occured again - yet). I am publishing the pose data at a high frequency of 50Hz and it seems that RViz somehow can't cope with that high publishing rate. Unfortunately, the problem was not reproducible - sometimes the pose visual got stuck sooner, sometimes later, and very rarely it didn't get stuck. I solved the problem by introducing an intermediate node, that receives the pose data at 50Hz and republishes it (under a different topic name) at a rate of 20Hz and now the pose display works like a charm.

I strongly believe that this is a timing issue, but unfortunately my understanding of how RViz internally works is too limited to deliver a tangible explanation.