Follow up: I gave up on the update, deinstalled the Hub -and the update- and reinstalled it using the full installer I found there: Unity Hub v2.0 beta is now available for download
I started the new Hub; I guess it’s working as intended but the window says “WINDOW TITLE” instead of Unity Hub Beta.
Like in the previous version, selecting a version of Unity for a project doesn’t hold until you start the project.