Dock buildings, at least in the 1750s, are missing graphics for seamless double height interaction. This applies to at least the passenger dock available in the 1750s, but almost certainly effects other docks.
They currently only have graphics for a double height ramp. If one places them on a single height ramp, which one can do and is encouraged to do as coasts are usually single height, then it appears that they float above it in an ugly way since the art is intended to match the double height ramp.
Here is the art file for the dock I am referring to. It seems the other docks also are missing the required single height ramp graphics.
My theory is that new double height graphics were not made during the double height transition due to an oversight. The current graphics match the double height ramp because that is graphically the same as the old ramp, which the docks were designed for.