This rectangle you see means that there is no decal material for the building and season you've chosen. Please open up the blueprint, details tab, and find "DecalPerSeasonPerTier". Each building tier needs to have this set up. When none is found, the game code reverts to default "Summer" season from this array. If there is none, it's a bad setup and results in what you see on screen.

Thanks. I've just copied material from generator that had one to the one that had none.