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I found that some of the maps in my in-progress SPZ1 will initially start as a black/white mess when loaded from the command line. Hitting escape will always fix this though. I really don't know what to make of this:
I took a quick look at Doom2.wad and found this happening in at least MAP01-MAP04 in gzdoom 4.12.2
The only difference is that opening the console fixes it (Sorry for the triple post)