Hi - was happening for me too with a mysterious badge count of 2. I followed a prior suggestion and it worked.
1. Switch to hosting
2. Go to Inbox
3. Filter on unread
4. I found two old messages
5. Read them and the badge count vanished
I think this is a relatively new problem; I don’t recall seeing the badge count like this prior. I’m writing this in late Oct 2021 yet the messages I found were from 2016… I miss things from time to time but I don’t think I’ve gone five years without noticing this 🙂
Hope this might help others too!