Imported brushes can have incorrect UVs if the textures are not power of 2. This most often happens when you converted WAD files to VMT/VTF/TGA with XWAD to bring in Goldsource levels.

At this time, the fix is to use Wall Worm Pro and make sure that non-standard textures (those not having power of 2 dimensions) have a .sizeinfo file alongsite the texture file. Wall Worm Pro will check for this file to properly resize the texture at the time of importing RMF/MAP/VMF. The .sizinfo files are generated by XWad for all non-standard textures.