App shows an unread message that does not exist, how do I clear it?

It is almost always sitting in Message Requests or Spam, those do not clear from the main inbox. Open that folder, read or delete what is there, and the count drops. Reading it once on messenger.com also fixes the phantom count fast.

If the folders are empty too, clear the app cache and pull to refresh, the badge is desynced from the server. That combo has cleared every stuck-unread I have had.