If we have learned nothing else from being an insider, it is you cannot know what MS is going to do next with these builds. Saying that this can't be RTM is like saying new builds are never released on Friday....
I had this kind of issue one time in the past only to find out that switching from fast ring to slow ring and back was helpful after restarting the system several times. It can be disturbing at times when you know nothing is wrong with your OS.
New in this build: if you had any customizations to the menu seen when you right-click Start, then they're probably gone. If that's ever happened before, it's been at least a year. Old in this build: Apps that require UAC and that are run from Explorer will probably (not always) leave the UAC prompt hidden behind Explorer. Exception: if you use UAC dimming, which I now do to avoid this stupid issue.
In my opinion, disabling UAC is not a good idea, sure it's a hassle when you want to run something, but what is good about it is it will not let anything install on your PC without your approval.
Seeing the UAC bug for a very long time. IIRC since 8.1. It gets worse if you enable launch folders in a separate process and is easiest to reproduce in program files.
I agree - UAC w/o dimming is still UAC though. Interesting, as a number of people talking about this only saw it spring up as of about a month ago. It even turned up in the release notes then as "fixed," which is a bit ironic, since before that mention I'd never seen it happen before.