Why is lower-unpegged line has lower texture scroll when ceiling in front of it raise

[ViolentBeetle]

I made a balcony of a sort that is meant to be sealed away by a shutter that opens when you cross a linedef. In order to better seal the room I made shutter adjacent. Which causes, for some reason, the unpegged lower texture behind it to scroll up when shutter is raised, even though my every instinct tells me it should be nailed to the floor. What is causing this behaviour?

[Kappes Buur]

It's difficult to visualize what you have constructed.

Either upload the pwad or at least an ingame screenshot of that balcony and shutter.

But the pwad would be preferential.

[ViolentBeetle]

I have redrawn geometry of the level and didn't save the copy, but here's a wad I constructed to demonstrate the effect. Simply make a step forward and behold unexpected behaviour.

peggery.rar

[Kappes Buur]

I really do not like DOOM/BOOM format, they are too limiting. But here are possible solutions:

 

peggery.7z

Edited April 29, 2020 by Kappes Buur

[ViolentBeetle]

Well, I figured out methods to fix it. I was trying to figure out what exactly is the mechanic that makes it move to begin with.