Waking both of a constraint's rigidbodies re-enables collisions that were disabled by the physics constraint You can also get this issue to occur by either shooting the two cubes around a bit, or by utilizing the Wake Rigid Body function on the static mesh components for the cubes.
Repro Rate: 4/5
Tested in //UE4/Release-4.27 @ CL#18319896 and the cubes did not collide after being woken, it is a regression.
Results: The two cubes collide with each other even though the Physics Constraint Actor is disabling collision for the constrained actors. (If they fall off and don't collide, move them around a bit to see if they start colliding!)
Expected: The Constrained actors do not collide with each other, following the constraints set by the Physics Constraint Actor they're bound to.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-148868 in the post.