Just to follow up . . .
I was up to 4 "bad" chips - out of ~50.
Using the same setup described in the 1st post, I did the same operation on another machine - same REL 22 installed.
I was able to bootload and program all 4 of the "bad" chips.
A mystery I'm not inclined to pursue, but just in case it happens to you or a loved one.