Spoiler:Look in the upper left and right corners, you can see HOM (black in the screenshot, since R_ClearBuffer is 1). Couldn't get this problem in ZDoom, possibly due to view pitch restriction.
There are no nodes in the Wad, I'm letting GZDoom generate them at run. However, building them with ZDBSP first had no effect.
If you need a sample map, one can be found in this ZDoom Forum post.