In GZDoom, extrafloor itself if solid-filled with the model sector's fade color.
Also, both these behaviours don't correspond to the wiki description, which implies fog boundary effect(?):
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4: 'Fog' effect. This is not real fog though, it just draws some translucent colored polygons on the outside of the 3D-floor.
Also, is there any particular reason why FF_FADEWALLS:512 and FF_RESET:1024 flags are not documented?