This isn't working. With my Unread folder empty I still find messages marked unread in my Inbox. Steps taken: refresh page, quit browser and restart. After this, with empty unread, trash, and spam folders I still find three messages (i.e., conversations/threads) in my Inbox marked unread from early November. There may be more. That is as far back as I looked.
Google Chrome is up to date
Version 78.0.3904.108 (Official Build) (64-bit)
Windows 10 Pro
OS Build 18362.418