This is very likely a regression due to recent changes in user list. When a new message is sent in a one-to-one PM, the unread count of All_PM button is updated immediately. But the unread_count of the user_button for that respective user is delayed for quite some time. Only after waiting, is the user button unread_count also seen to increase.

The possible reason for this behaviour is due to the user_list updating which happend every 60 seconds conflicting with the normal UI updation.