Creating a polyline is working fine : it appears correctly in 3D view, without any artifact.

I just created a plan with 5 rooms (17 walls) -> no artifact
But... I've added another room, still ok, and then another one : artifacts appeared on the ground. It's different from the other artifacts which are going in all 3D directions (x,y,z) : these ones are only on (x,y) plan.
Moving the room (the floor, without the walls) outside of the 'house' will make the artifacts disappear...

I can send you some screenshots if needed.