yes that area is somewhat weird on RO. perhaps also notice the "lake" has no water in it. i tried to make that with water. impossible to do as files are weirdly mapped so it just cant be done really. i suspect this is a "void" area. and perhaps is why this is not possible in that specific area. (not sure about this though)
The slab is set at zero and can't go any lower. Non-concrete objects by default don't get placed exactly at their set height, they are 'dropped' from a bit higher and then placed where they hit the ground.