When spawning Pawns with a StaticMeshComponent with SimulatePhyscis=True, they occasionally may not simulate physics. This can be reproduced by simply repeating the respawn. The frequency of reproduction is about once every 2 to 10 times.
If it doesn't simulate, it can be reproduced by re-setting SetSimulatePhysics(True) on the StaticMesh.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-161665 in the post.