I got it working…
But Im not 100% on how.
I think it was setting recastnavigationmesh actor to rebuild at runtime?
But Im not actually telling it to rebuild anywhere, so I am assuming it rebuilds automatically periodically?
Sill not sure how this all works so Im going to mark this as a comment and not an answer.
Player in top-down example uses navmesh for movement, so if you want to spawn traversable meshes at runtime you need to have navmesh runtime generation enabled. If you do then navmesh will automatically update in response to spawning new collidable actors.