Sun texture becomes replaced with map texture

The Bug

After a normal night cycle, the sun rose with the texture of one of my existing maps, seemingly the first one created. This was a random occurrence. Not a game breaking bug, just odd. The map it took from was placed in an item frame. After sleeping through the night, the sun is still this texture. 

I tried reloading my save (doing "Save & Exit" then reloading the world) and it just got worse with now an unrecognisable texture.

The moon is normal.

Steps to reproduce

Currently unknown.