MatroskaLib has a typo within MantroskaConst defining the first aired date for a season.

see https://github.com/MediaPortal/MediaPortal-2/blob/dev/MediaPortal/Source/Extensions/MetadataExtractors/MatroskaLib/MatroskaConsts.cs#L37

Since the first aired for a season is not available within online resources it is unlikely that a mkv file has it. Even if one would have it, then it would be properly tagged and also have an episode release date. In this case MP2 would not use the season release date anymore.

So better option might be to remove it.

h4. Additional Information
Update docs if done: http://wiki.team-mediaportal.com/2_MEDIAPORTAL_2/Contribute/Development/3_Concepts/Metadata_system/MediaAspects/SeriesTags