A hill tile has been replaced with emptiness and too steep borders in a square all around it. It's just the single tile; I tried approaching it from all sides.
I had not visited this tile before, but I have some hardware issue with my computer (probably the PSU) that causes it to turn off now and then, so possibly I was in a nearby tile and some data was corrupted -- the hardware issue has happened a few times while i was searching for this ram and I didn't note down the locations I was in when it happened.
I have a save available in case this is an actual bug and unrelated to my hardware problem.