When you hide a topic, the notifications relating to that topic are deleted. However, if there are notifications still queued to be sent (eg because there were a large number of followers), those notifications still end up getting sent, so users receive a notification about a post that they can't see, and the notification doesn't ever get deleted.

Thank you for bringing this issue to our attention! I can confirm this should be further reviewed and I have logged an internal bug report for our development team to investigate and address as necessary, in a future maintenance release.

 

Just a quick note to let you know this has been resolved in the latest 4.7.4 release, which went out a short time ago. If you are still experiencing this issue after upgrading to that release, please let us know.