Plank wall tops do not attach is a plank wall window is below.
When I tried to place a Plank Wall Top on top of a Plank Wall Window it would place, make some particle effects and float up a bit completely detaching itself from the grid, I have tested this with all other wall types and it doesn't happen to them. I have attached a picture that shows it not being attached. And yes it is replicatable in vanilla and I am on version 0.7.1 of ME.
This bug doesn't replicate with the other wall types either just happens with the Plank Wall Window.