I suggest replicating the parts to another game where you can Team Test, as a Team Test is the most accurate thing after Testing the game itself.
And locate the glitch by doing so.
(Btw, check on Team Test/Game Test if this bug is still here or if it’s just a Roblox Bug).
I’m not familiar enough with the Roblox CSG service so someone else more fluent in that can help out, however I’d recommend taking a look at the CSG articles for unioning in-game. As far as my practices go, I generate my unions outside of scripts. I would also recommend looking into errors that might not be CSG-related.
Unfortunately, in my case, I have to generate the Union inside the script, since I have a flexible distance between windows.
Also, Studio is not generating any error and, as you can see in the OP, the wall parameters are ok, visible, so it’s a mystery, but it has to be solved before my game is launched.