Recently added also displays the “Show” poster.
The only way you can see the season poster on the home page of Plex is to add two or more episodes for the same season, I posted screenshots, and my explanation of the random logic Plex uses to display posters in the feature request I made years ago here:
I’m pretty sure I even made a similar “feature request” a year or so before that (the system has changed a few times over the years) and was told by a Plex employee it doesn’t make sense to show the season poster. Sort of the answer here. 
It seems like a simple and logical request, and I’m not sure why it’s not the default method.
