When I build a multiblock tank (formed from multiple tank blocks next to each other), it doesn't always show the water level in all of them. Sometimes I can have one filled, put an empty one next to it, and one is full but the other is empty, rather than both being considered a single half-full tank. Also, putting 3 in an L-shape shows as empty, even if one or all of them have water.

Edit: Using a20 which was just released, it partially shows water, but adding water does not update all connected tanks unless they are in a square shape.