That's okay to keep the thread here.
I can confirm your issue, however.
Registration appears to be broken in the current build (19.0.117). I just intentionally unregistered my copy of MC, and then installed the newest build. It is just as grantb indicates. You can install the key, and it appears to work, but it doesn't "take".
It does, however, use up one of your restores. By the way, grantb, don't worry about the restores. They can reset those. (Speaking of which, I just used up a bunch of them too, testing this out.)
Also, grantb, to solve your problem for right now: If you install the older 19.0.103 build, it will let you register the application properly. Then you can upgrade it to the current 117 build, and the registration "sticks". But you can't register it new from within 117 right now.
Proof (I was going to make an installation demo, but then it turned out to be true):
https://vimeo.com/87331668