Drawing a polygon starting and finishing at an existing vertex produces a solid box

[ioan-chera]

If I start drawing lines from an existing vertex, finishing the polygon at the same vertex, I get a solid (void) wall, even if I do it clockwise as if to generate a sector. I need to check if this is a bug, and fix it if so. Doomworld link: https://www.doomworld.com/forum/post/2823442