If I make a floor 1 unit taller than the surrounding sector, it seems to be getting rendered almost as if it was the same height as the surrounding sector and that the edges of the higher sector (which are correctly applied to the lower side defs) look almost as if they are 1 unit tall textures applied to the middle of the sidedefs. This is GZdoom only. Zdoom displays it as intended.
Screenshot time:
GZdoom:

Zdoom:

Test WAD attached.