I got a 32x32 or 64x64 decal, both render fine until I replace it with a high-res graphic in the pk3's hires folder. Then it renders really badly. At first I thought it wasn't rendering at all but it actually was distorting it over quite a few parts of the walls.
Spoiler: ScreenshotsIf you need a test wad, I have supplied it here. MAP01 is the non-bugged decals. MAP02 has the bugged decals.