Year-based seasons create a spurious "Season Unknown"

[odradek]

I file a tv episode like this:

Arena (1975)/Season 2003/Arena - s2003e02 - The Life and Times of Count Luchino Visconti.mkv

along with a tMM-generated .nfo file which includes the lines:

<showtitle>Arena</showtitle>
<season>2003</season>
<episode>2</episode>
...
<episode_groups>
<group episode="2" id="AIRED" name="Aired Order" season="2003"/>
</episode_groups>

and I scan all libraries (no metadata downloaders are enabled, so — I think — everything should come from the .nfo file).  

In Jellyfin, on the show's page, I find Season 2003 and Season Unknown, both of which contain the same episode.  

I want Season 2003 to remain and Season Unknown to disappear.  How do I make this happen?