Excavator tries to mine the air

I've placed a sloped mining designation so that a good chunk of it is above the terrain. This should mean nothing tries to mine that part. However, there's a bush under it, and the excavator wants to mine that designation. But it tries to travel to the designation surface (in the air) and complains that it's unreachable.

Using the remove debris button (Ctrl + U) will clear that, something to fix it now. But yeah that is odd behavior