I recently noticed on my Xiaomi Mi 9T phone (android 11) that the AquaMail badge count was showing around 20 more unread messages than there actually were inside AquaMail. I thought the issue was related to my device's notifications settings, but then noticed the exact same inconsistency on my tablet (Samsung Galaxy Tab S4 on android 10).
This morning the badge count displayed 47 unread messages on both my phone as well as my tablet. I took screenshots before and after opening both AquaMail apps. Both apps showed 47 unread messages on the badge counts, with both showing the actual number of unread messages at the top = 26 when AquaMail was open.
Here are screenshots showing badge count and actual number of unread emails in the app.