Adobe InDesign v16.1 (12 Feb 2021 release)
Windows 10

This bug has appeared again, after disappearing for a couple of versions. But it was in a previous version a few months ago.

When the Book properties panel is open, I make changes to my Book file (.indb) and then close the Book panel or select "Save" from the panel menu. It asks me do I want to save.

Yes, I want to save... but the dialog box is acting like a Save As... It's asking me for a file name, then asking me to overwrite the existing file, instead of simply saving the existing file as a regular Save should do.