I did some more testing and found out that the problem only occurs with RAW+JPG pairs created in the camera.
1) If PM starts with "Combined Images " selected, no histogram is displayed.
2) If I then change the viewing setting to undo "Combined Images", the histogram is displayed both for RAW and JPG files.
3) If I now select "Combined Images" again, the histogram stays visible.
So I have a workaround but it still looks like a bug...