(with Vavoom 1.26 win32 and Doom/Hexen)
Strange, if there's a 3D floor with a 2s line having some middle texture, that texture is drawn on both sides of the 3D floor; on the top and below it.
If the 2s line is inside the 3D floor sector both sides of the 2s line's midtexture are drawn multiplied. But if the 2s line is just bordering the 3D floor sector, then only the 3D floor sector's side of the 2s line's midtexture is drawn multiplied.
A demo level here:
<!-- m --><a class="postlink" href="http://www.geocities.com/gearlost/misc3/midtex_3d.zip">http://www.geocities.com/gearlost/misc3/midtex_3d.zip</a><!-- m -->