TL;DR
After i filled some water with the vanilla landfill, the area is now bright green grass, even if it was in a desert terrain.Description
I tried the vanilla landfill and i find it rather strange that the filled areas have always the same terrain type.I don't know if this is an bug or intended behaviour. Also i don't know how this is implemented. If a mod can think of a better place than "General discussion" then please move it.
Is the map generated and every tile get its terrain type assigned once at the beginning and the information is dropped afterwards? If yes, would it be possible to save the terrain type for an water tile too, so if you replace water with dirt, the dirt can choose the right terrain type?
Example images