Actually never mind about the arch. I see it now.
The version of your mesh/CSG that the physics system uses for collision detection is often lower-fidelity than the visual mesh. If you do a lot of CSG, get this plugin to easily toggle on and off the setting to show the convex decomposition of your mesh (the physics geometry you get for collision mode Default). You can see your mesh’s Hull and Box options visualized with this on too.
Often, if you use CSG to cut holes in a part, they will not be the shape you expect in the physics mesh. They can even seal up entirely (something you’ll see if you try to make a lot of window holes in a big wall with negate operations).
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I’ve had this exact issue with unions, They can be touchy when it comes to collisions after negating an area.
I can’t be much help as to exactly how I fixed the issue but if I remember correctly the negate part should have can collide off and you either need to select main first, then negated to union or the other way around.
It works but can be a pain in the arse. Lucky thing we have Blender and Maya huh