So, I finally decided today was the day to start adding cubemaps to the artpass map. I popped a bunch in where I thought they'd be the most useful, recompiled, turned specular off and tried to run cubemaps. It displayed the usual animations of the cubemaps' faces in HDR, but then I got about a dozen or so "Tried to load a non-VTF file as a VTF file!" errors. And, lo and behold, when I turned specular back on and reloaded the materials, I got pink squares. So apparently the game took the screenshots but then tried to save the cubemaps as non-VTF files or something? I'm clueless. And apparently this has never happened to anyone else before, because entering the phrase + "Tried to load a non-VTF file as a VTF file!" + cubemaps in the Google got me nothing related to my problem.