Hello,
This seems to be working as intended, if I am understanding your issue correctly. In order for the collision to function properly, you’ll need to ensure that at the very least your mesh is parented to your collision box. In typical blueprints that I create, I try to ensure that the collision I want to use is set as the root, as this is intended workflow. When I followed your repro steps that you included in your linked post, moving the collision box to the root component slot ensured that I was able to get the collision to function as expected.
If you’ll notice on the default projectile included in the First Person Template, the collision is set as the root, which is necessary for it to function properly.
Also, I’m not sure what you mean by “break ALL collision-checking for this object except for pawns”. What sort of behavior are you seeing with your pawns?