Since you can get a workaround with a next song button, so I just can say it a bug in mala. I know bass.dll my self, since I also use it in Arcade Music Box.
I guess: The handler for the song is gone for some reason, but mala dosent check for that, even bass.dll may have returned a error. That error is not checked by Mala, hince it dosen't skip the song, as it should or restart it.
Does mala also free bass.dll before running third party app (wich it should)? Otherwice it not a problem to reinit bass and use it again (since song position can been checked before closing).