Title.
It seems the devs of Twilight Forest never bothered to implement their door texture code correctly (and outright refuse to fix it for no reason), resulting in missing textures that cannot be fixed via resourcepacks. This is not caused by other mods, by the way.
Combine this with their triple-licensed mess they use, it makes fixing this impossible since forks are a no-go.

(Also, these doors (in recent versions of TF at least) are now in the TF tab itself, not in Redstone. This should probably be done too, if wanted.)