This is about the "ramp inner corner" block shape, i.e. the penultimate shape, right before the pyramid, shown abandoned on ground in the screen shot below.
It seems to me that it lacks an orientation. For instance, in the screen shot below, I could not find a way to fill the two corners of the sloped frame around the door (nor the two other corners of a (hypothetical) whole frame).
This shape can be flipped upside-down to fill a down-looking slope corner, but not sideways to fill a side-looking slope, like this one; note that the ramp shape can be flipped sideways, in addition to up and down.
Didn't I look hard enough or it is really missing?