Stalagmite slope is broken when you are too far from it

[GLide KS]

I don't know if this is a bug or not, I don't know if this should be in "Editing Help", I don't know if I'm doing something wrong, but when you are too far from a stalagmite slope, shows a missing texture, I put the textures at frontside and backside, but when you are closer, it doesn't break.

Also, here's the linedef structure if you want more details, just in case if I'm doing something wrong.