If you get this issue this is down to the previous issue with order in layers.
To solve this go to the “Base” gameobject we created and under the TileMap Renderer change the order in layer to -2 much like you may have had to do with the previous grass sprite we had as a temporary background.
Stephen does get this issue with some of his trees and later in the lecture he will solve this with moving it to -2 as well.