User blog comment:Maxfield, the Garryfish with PTSD/Broken Terrain Bug/@comment-65.158.83.46-20170424174410

I had some disappearing floors in the ILZ a couple months ago, but they came back when I saved/reloaded. I assume since you're posting here you tried that step already?

If so, next step might be to look into the terrain batch files (they were frequently discussed for a while since deleting them helped fix lag, but it seems plausible they could be corrupted in a way that might cause this... but I'm just guessing at this point)