An old version which I've still installed (2.27) indeed restarts the file when depressing the key for "next" (with only one file on the playlist).
I'm not sure whether this was an intended feature, so I can't say if the fact this doesn't work so anymore does represent a bug!?
You could - of course - use the bookmark feature.
However, if you simply want to restart a single playing file, it is a much more easier solution to simply depress [V] followed by depressing [X].
