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They have been put there as placeholders and make it's easier to add new tiles to the tilesets for any future update.
Make sure, if you're using Unity 2019, that the 2D Tileset Editor package is installed.
It's a common problem and can have different solution. Try creating a Unity Sprite Atlas and pack the tilesets with some padding. If that doesn't help read more suggestions for solutions in this post.
The scene was created using one tilemap grid for each room. This is for flexibility and allows you to handpick if there is any specific room you would like to use. Ideally, you would use as few tilemap renderers as possible showing inside the camera at the time.