The error happens in version 6.3. Very strangely, the wall 2 is joined to itself at its start. I wonder how that could happen, and if you find a way to reproduce this issue, I would be happy to know it so I can fix it.
Like I said, I updated an old project (6.1.2) and I saved it under 6.4.I closed sh3d then I re-opened the project, the walls (not all) became unjoined...
The size of your SH3D file comes from the texture image you used for the ground.
OK, TY. 
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Evil progresses when good people do nothing!
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SH3D 7.1 and nothing else - W11 64b in 4K
