Gravity / Physics on ProBuilder objects

This seems like such a simple thing, but I can't figure it out.

Without ProBuilder

New Plane (10x10), New Cube, raise cube above plane, add RigidBody, click Play. Cube falls to plane.

With Probuilder:

Open ProBuilder Window. New Shape (Plane), Build. New Shape (Cube, shrink to 1x1), raise Cube above plane, add RigidBody, click play. Everything stays in place. Tweak settings (Mass, Is Kinematic), nothing changes.

I saw on an old thread something about static and moving, but can't find those settings now. I imagine this is something trivially simple, hoping someone can point me in the right direction.

Tac

 

Hi there, very late but I just had the same problem.
If you click on the object and look in the inspector in the top right you will find a tick box called static.
You just need to untick that and it will work.