Especially now that the plugin has been renamed “Unique note creator” this behaviour very much seems like a bug to me.
If I set my prefix to be YYMMDD, auto-incrementing it is wrong. The intent is to create a note for the date it was created. I can’t think of any reason why “use tomorrow’s date if you’ve already created a note today” would be the right behaviour. If you create and rename seven notes, the plugin should not create a note dated for next week (or month if we’re at the end!) because creating a note with today’s date will still yield a unique name.
I can understand some way to add uniqueness if there is a note conflict, but in my case (and I suspect the OP’s case) there is no name conflict, but even the Zettelkasten method (described at zettelkasten.de at least) does not make up a date. They add digits by appending, not incrementing.
+1 to this being a bug