Can't find cause of badge

I just upgraded to the new UI.

I have multiple accounts with multiple folders in each account.

I need to use badges. All the messages in all the folders in all the accounts are read, except one inbox has 36 unread messages. I’m not ready to read them all yet.

The app icon has the badge “1” on it.

How do I acknowledge what the 1 is telling me so that I can get rid of it and see new badges when new messages arrive in that inbox and the inboxes of other accounts?