unknown textures on startup, no problems in GZDB

[Lollipop]

Hello once again, I am having this:

Now, map analysis says there are only a few missing textures and nothing else (these are slopes), so I do not understand what this is coming from.

This is a major problem, as my map takes several minutes to load because of this wierdness. (and also managed to make paint lag like a bitch too)

[Dark-Assassin]

What textures are those linedefs supposed to have?

Custom or stock ones?

[Lollipop]

I am currently building the map structure, and have only used stock doom2 textures, and have not even loaded anything else.
The wierd thing is, that one line tell that it is using a texture on the back, when it does not. The one I think of is a lamp in the ceiling with a border, and the game think that there is a texture on the side of the line that face the void.

[Dark-Assassin]

That's really weird.

[Empyre]

I'd put a texture where it thinks there is a bad one, and then remove it. That has a chance to fix such weirdness.

[Lollipop]

I just want to tell that it does not happen in zdoom. At all. So I find this to be a zandronum specific issue.
I sldo noticed that all those lines actually did miss a texture, instead of having the "-" texture, it simply had a blank field. No fucks given from zdoom anyway, while zandronum goes "the fuck is this shit?"

EDIT: I have noticed that some do have the -, but they do simply not behave, even if actually given a texture, it still says unknown texture ""

EDIT2: I have just tested with normal doom builder with skulltag doom in hexen format, and the error dissapeared, I am going to test more with formats and see if I can nail the issue.

EDIT3: I have nailed the error, it is as follows, with tests included:

Error does occur when I use:
GZDB with UDMF format
DB with UDMF format

The error does not occur when I use:
GZDB with hexen format
DB with hexen format

Testing with zdoom have, as already mentioned, no issues on startup at all and works just fine and start instantly.