There is a problem in some of the Doom ports with missing floor textures with sky or void space appearing instead, because these source ports require an additional GL information in the WAD files. This can be fixed by using the glBSP utility, which adds this required additional GL Nodes information to the WAD files, solving the problem with disappearing textures.
Unpack the 'glbsp_win_224.zip' archive and run 'glbspx.exe' file.
Click 'Browse' for 'Input' and locate a WAD file you'd like to convert.
Set the same file path for the 'Output' field, so the file will get overwritten. Click 'Build' to start building GL Nodes in your WAD file.
The warning window will appear, questioning if you really want to overwrite the source file. Although this window warns about possible slowdown, it didn't take much time on my machine.