Well, it looks like I'm not out of my gourd after all; seems several users
have had a similar experience; there simply is nothing there to explain
what's wrong. The weird thing is, I was redefining the section; it's nothing
more than a rounded rectangle, which originally had two of its sides
coincident with two existing edges on the part. I removed the coincident
constraints so that I could extend those two sides beyond the edges, and all
of a sudden Pro decided it needed a dimension on one of those two segments,
even though I had the two overall dimensions and the radius, all points were
symmetrical across centerlines, and before I stretched it out, it was a
closed loop (and it still appeared to be after the change). I had tried to
let it go with the extra dimension, but the feature kept failing. So, I was
trying to get rid of the unnecessary segment dimension by trimming the ends
of the line to the arcs, but Pro wasn't having it. I finally just deleted
the segment and drew a new one between the ends of the arcs, and everything
was just fine.
Thanks for all the suggestions/comments.