Returned a book to the library but it's still checked out

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I had a book checked out from the library that was due November 5th. On Tuesday, I dropped it into the bin at the library—that's how returns are done now due to covid. I just received a reminder email from the library telling me that my book is due November 5th. I was surprised to get this email since I already returned my book. I logged into my public library account and noticed that the book is still shown as checked out. I wonder why that is. I hope the book didn't get misplaced by them or the return didn't go through. I checked out three other books and never had this issue when I returned them. I wonder what happened. I may have to call the library to find out what happened. I would go there but the only thing we can do is drop books off into the bin to return them and/or pick up books already checked out for us and then leave.