When adding media to playlist, parole immediately stops the currently playing media (which is still in the playlist) and skips to the newly added music/video and plays it. This behaviour is desired if you open a file, but not when just modifying the playlist.
It looks like you have the "Start playing opened files" option turned ON. Please see http://docs.xfce.org/apps/parole/preferences for more information. Please let us know if you are having this issue with this option turned off.
Thanks Sean, that option was indeed on. With it off, the currently playing file continues in both cases, whether by adding new items to the playlist or by opening a new file. In my opinion, it would make more sense if per default opening would play the new file and adding would not, but I’m afraid perhaps that’d be a different issue.
Closing it then.