For seasonal tiles you only create 4 Seasonal tilesets

Name them with    season_tilename

And reference only the  spring_tilename one in the Tiled editor.
While naming it   ztilename   (always a "z" at the start)



Edit 2021: turns out, you name them z_  For a loading order fix everyone is using.
Some vanilla tilesheets are called "Untitled" and the custom thilesheets loaded before the letter "U" are being overwritten.

So, the z or z_ has nothing to do with being seasonal or not. Its just a community fix.   