So it turns out I was misguided in thinking "I don't need vertex shader 3.0 for this mod, only the pixel shader needs an upgrade!" back when I first added the parallax mapping to the mod. This is because 3.0 does not play well at all with older shader versions. This was disguised by the fact that my PC has a Nvidia card in it, and for whatever reason, Nvidia seems to silently convert mismatches such as these so that a vertex shader 1.1 and pixel shader 3.0 compilation mix doesn't break. This wasn't the case on AMD cards, which basically turned off the shader entirely upon seeing my unholy combination of versions. I got into the weeds for a while because it mimicked an alpha testing error (discarding pixels with transparencies according to certain criteria), but thankfully after lots of useful system spec info sent to me by users experiencing the problem, I was able to pinpoint the problem to AMD cards, and ultimately to the version issues.
There should be no visual changes in v0.5 (the version with the fix), aside from the addition of actual working visuals for AMD cards, so comment if something looks off!