I made a video for you and did a few flyovers of the map so you can see what it is doing. Turns out this effect is happening in either perspective mode or orhographic mode for the camera.
It seems to be a sprite issue, as here even the preview the problem can be seen. It is kind of like in 3D graphics when the normals are not turned the right way. I have attempted rotating the sprite to no effect. On my side I used a different paint program to export the sprite, also no effect.
It would be tedious, but one could create a really thin cube in something like Blender, and then have a texture for just one side. But then you would lose the transparent properties of a .png wouldn’t you?