I found the problem with my heads, and as usual it was my fault
While using the checking springs, I noticed a suspicious amount of movement in the retainer. With the valve in my hand, no spring, and the retainer pulled tight against the old locks - it wobbles back and forth. Not a huge amount, but enough.
If I were more experienced at assembling heads, I would have noticed it. Oh well, live and learn.
A comparison test with the new 614 locks and the new +.050 locks, and it's gnat's *** tight. I tried several random combinations of old locks not showing visible wear or damage, and all of them let the retainer wobble.
And if that wasn't evidence enough - after searching my bench and the surrounding areas (for the third time)... I found another brand-new, clean and sealed Comp bag of +.050 614 locks. Those are the ones that Dwayne sent me, obviously.
So the inescapable conclusion from the above is that somehow in my excitement to get the heads together and on the engine - I had put the old 3/8" locks on my new 11/32" valves.
I'm lucky that only one valve dropped and that it was at low/idle RPM. Could have been a lot worse.
Not only that - that also explains why my installed height was .050 lower than I thought, because Dwayne had sent +.050 locks which of course is exactly what I need!! So instead of 130/430 lbs, I was running more like 155/455. At least the Mini-Express cam and mushroom lifters look like they survived the ~1 hour of run time, and none of the valves were way out of adjustment, even the ones with loose keepers (not the dropped one, obviously).
Lesson (re)-learned, and I shouldn't have needed a reminder after so many years- anything you don't check (preferably twice) is going to bite you in the butt. Murphy's Law reigns supreme...