When a conversation without any unread messages is opened, the unread message counter in the page header is decremented (erroneously). It stays like this until the page is reloaded, at which point it resets to the correct value.

Demonstration on https://sandbox.moodledemo.net/ (see attached screenshots):

1. Screenshot 1: Student Sam has one unread message in the conversation with Terri Teacher
2. Screenshot 2: They click on the conversation with Admin User (not Terry Teacher). Note that the unread message counter was decremented to 0 and vanished.
3. Screenshot 3: They go back to the conversation list (without reloading the page). Note that the conversation with Terry Teacher is still (correctly) marked as unread, but the unread message counter in the page header and the list header are gone/broken.
4. When they reload the page, it resets to the state seen in Screenshot 1.