This is backface culling; for performance reasons, Godot will automatically avoid rendering object faces that the player isn’t expected to see, but occasionally this is not the behaviour you want. In the StandardMaterial3D Inspector for this Mesh, look for Transparency–>Cull Mode and set it to Disabled, which is the intended mode for two-sided objects.
I don’t know if these tiles were designed to only be seen from one side or if this is just some kind of oversight, and I can’t download the project to check at the moment because the gateway is down. If it is a one-sided model, it probably won’t look the way you expect, but changing Cull Mode should still get you further ahead.
If Cull Mode doesn’t do the job, you may have to place another layer of tiles upside-down to act as a ceiling.