rrr

Ahhh...the Recycle Bin. Yes, the 2 placeholder 'default new database' entries that KeePass makes WERE in the Recycle Bin and that is what it was matching (since one of them is using "User Name") Tricky tricky. That solves the puzzle. I guess autofilling from deleted (but not emptied) entries could be argued either way - include or not include (because they have been deleted). But the 'bug' is now explained.

screenshot of a new DB, blank (I deleted the 2 placeholder entries that KeePass creates for a new DB, then saved the file). Then shown creating a new entry, typing "use" and "User Name" pops up.

Nope. Already anticipated and checked :-) There is no entry in the database with a user name of "User Name". Did you try the reproduction on your own database? I gather you dont have a user name for any entry of "User Name", but when you start typing in that blank it will autofill an option for "User Name". (You can also create a new database, which starts with 2 placeholder entries, and yes one of them has a username of "User Name" (I think this is starting to zero in on the bug.) Then delete both...

autofill for "user name" field on an entry included "user name" as a choice