Birdsmouth for rafters on high end of shed roof not correct

[chungus]

I'm building a single pitch shed-style (lean-to) roof and Home Designer does not correctly seat the rafter on top of the high wall with a birdsmouth. The low end is correctly shown, but the high end just shows the full rafter without any cut or adjustment to sit on the top plate- it only sits on the corner.  

[chungus]

Shown is the small gap created and this occurs on every rafter. The end wall top plates matched the pitch, but I just wanted a birdmouth to sit flush on the sill plates and automatically lower those top plates. Is there a way to get this to work right?