HDP wouldn't allow me to cut'n'paste an attic wall plate into this wall. I tried in both 'Plan' and 'Full Perspective' views. I decided Jeff was probably right and there was just something about the structure or placement of this wall that confused HDP. So I decided to save a copy of the plan, delete the attic wall altogether, and redraw it. That worked, and I got this:
BTW, I was following this tutorial to create the Dutch Roof. I used the "Manual" method.
https://www.chiefarchitect.com/support/article/KB-00749/creating-a-dutch-hip-roof.html
fberus's post in Attic Wall Missing Bottom Plate was marked as the answer