Thanks for the report, theG!
Just cleared up the field using mp3tag. In my collection, it first started with albums analyzed in May 11. It might have started 2 days earlier, but albums in May 8 were clear.
Open those tracks in mp3tag> select all and right click> Extended tags> find a blank field at the top and remove it> save.
[edit] No, it was earlier than that. It first started in May 3.