weird line on a transfered sky

Hi, we have a map for prboom-plus -complevel 9 that uses sky transfers but for some reason a weird line appears on the sky even though everything is fine with the texture itself as far as I can tell:




What could be the cause of this?

Could be a map or game-related glitch than a graphical one.

Hmm, I noticed that the glitch appears only if the map is in the first episode (map01-map11). I guess SKY1 somehow interferes with it, not sure why.
Here is the wad in question...

Might be an engine bug, the sky looks fine in glboom-plus and Eternity.