Hi, psills and welcome to the forums.
This is indeed a somewhat very rare occasion in WMP12. We already had a user with the same "issue" some time ago, use the forum search to try and locate the topic and see if it was resolved. As far as I know, there is no obvious option/setting in WMP that can resolve this, as most users generally have problems with adding album art tags into files through WMP12, or having WMP read them in the first place.