I can't find similar thread, so maybe I'm first to reporting auto-terraforming bug.
I noticed that Simutrans can terraform ground while placing new curiosities, but this behavior can cause bugs. I igonred few of them, and there were the same errors like this.
There is a slope next to the river. Game removes the river, extends the slope and then places a building.
EDIT: when I tried to remove incorrect slope, program returned text "Clear field before using slope tool". Under slope was the river.
I guess this building was built by simutrans? and not by yourself using the editor tools?
Edit: I have no clue what could have caused this. Such buildings are always built on vertical slopes if necessary :question:
MAybe the river was added later? Then the terraforming could have failed partly.