It sounds like you may be using the Gutenberg plugin, and are using the experimental new widget editor it comes with, as the steps you describe match it, but not the current WordPress beta.
For bugs you may discover with the Gutenberg plugin, please use https://github.com/WordPress/gutenberg/issues
I can say that I think I’ve already spotted your issue being reported at https://github.com/WordPress/gutenberg/issues/26300 though.
I’m marking this topic as resolved, as it is not related to the upcoming release of WordPress, and also because it has a ticket associated with it already.
Everything you said: true. Thanks, and sorry to bother!
No bother at all 🙂 Thanks for reporting it, it’s not always easy to know the right place for all these bits and pieces