Further tested and found what caused it:
"Full screen" selected under Preferences for
Playlist
Note: "Enqueue as default action" NOT selected under
General > File types
Also "Always start in full screen mode" NOT selected under Preferences for
Video [General]
I would consider this to be a bug! (When I simply load and play an individual movie file, I wouldn't expect a setting of only a playlist-preference to be of influence on that.)