The floor/walls have a convex hull collider, which sort of “shrinkwraps” a collider shape around the mesh. Since the floor and walls are one mesh, the collider shape is wrapped around the whole thing. As a result the dominos are actually inside of the collider when it starts which is why they’re going haywire when it plays.
I recorded a clip to show what happens when you drop the dominos from above. The way they land on the collider and slide off of it reveals the shape of the convex hull surface.
There are a couple of solutions. You could change the floor/wall collider shape to “Mesh” which will make it the same shape as the object, or just make the floor a separate object and remove the collider from the walls.
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