Here’s a gif showing the bug:
Let’s call the cube on top C1. The cube below is C2. The cube on the right is C3. They are just 3 cube static meshes that have simulate physics enabled. I first attach C2 to C1 and then C3 to C2, and after detaching C3 from C2, you see that the collision of C3 still exists in the welded mesh that consists of C1 and C2. So the collision of C3 is incorrectly left over where it would have to get cleared. C3 itself still collides correctly with the world, so the collision is duplicated and now exists twice in the world.
The exact steps for reproducing this are:
- Attach C2 to C1
- Attach C3 to C2
- Attach C1 to C0
- Detach C3 from C2
- Detach C1 from C0
C0 is a static mesh that is not simulating physics. Just a regular static mesh.
I have reproduced this in a clean test project (4.15P1), here you can download the project: