I don't have the problem when I select options such as "all" or type in a number, just when I select "-unspecified-"
That seems perfectly normal to me. And are you sure that something changed about it ?
When you edit the details of a series DVD, you can say which seasons are on it. "Season 1", or "Season 2", or "Season 1 & 2", or "All" for the complete series. If the DVD doesn't hold an entire season, you can still list the episodes. And if you specify nothing, no season nor episode information, that means that the DVD hold various (unspecified) episodes.
What's the problem ?