Harvey Did you reboot after updating?
Yes, apparently I did. But this time, I watched very carefully, and Etcher did what you said you saw.
The password prompt window flashed onscreen for a few miliseconds, then Etcher grabbed focus. I didn't notice that the first time. Probably blinked, or looked away from the screen for a second just at the right time.
By moving Etcher, I uncovered the authentication window. Sounds like Etcher needs a bit of fixin'.