Sector over Sector Bleed over

[Desfar]

Inspired by Toenails VOD mod, im working out how to use sector portals more.
However with this set up, the night sky appears semi randomly based on an unknown linedef.
Is this just a quirk I'm not aware of, or am i doing something wrong?

[Desfar]

Semi Resolved.
Fixed issue, but dont understand the why.
Posting the resolution for any hitting the same issue.
This sector, the flat section of the Arch has a single line def that would manifest as the sky texture.


Unsure why, but by making the arch have no COMPLETELY flat sections i no longer have any mystery textures appearing.

[Kappes Buur]

Even with the best screenshot it is difficuld, or impossible in some cases, to troubleshoot a problem.

Your best bet for help is to upload a map showing the problem.