Weird. I clicked the upgrade link in the screenshot above which reported no updates needed. Then I posted the message above. After that I went back through the Admin|Upgrade menu option which then reported I was missing a commit from 4 hours ago, even though I’d only updated 10 minutes before. I applied that update and I’m now on 3.2.0.beta2-dev and the critical update warning had returned to a nice green smiley face.
So the problem fixed itself but I don’t understand why things happened the way they did.