Steps to replicate:

1. Open a talk page on mobile using user account A.
2. Subscribe to a section that you are interested in.
3. Using another account B, post a comment under that section.
4. Log back in as user A.

Observe that, if you had unread notifications from before, despite receiving a new notification the counter on the bell icon did not get updated. And, if you had no unread notifications the bell icon stays unfilled making it easy to miss the new notification from the subscribed section completely.