So currently I'm making a wad that is
supposed to look like it was made by an amateur, i.e. missing textures, non-symmetrical rooms with uneven lengths, etc. I'm using UDMF(which I've pretty much gotten the hang of) as I have more creative variety with it than I do with the other formats, and this is my first problem with it. Did I do anything wrong with the usage of F_SKY1?
If it helps, I was trying to make an open area with stairs and pillars for the player to spawn into when I encountered that issue. I have no clue how I encountered it.

In-game

In visual mode