I can’t remember exactly when it started, but I’d say around the time automatic bumps were implemented not too long ago. I started receiving notifications from random posts in random topics that were not active anymore. By random posts I mean read posts in the middle of the topic, not necessarily the latest. By random topics I mean open topics without recent posts.
This is more an annoyance than a grave bug, but I suppose it’s confusing for people who receive notifications from old posts they probably forgot about already.
Also I have no clue how to investigate it, whether it comes from plugin interactions, or if you have reproduced it. So I’m telling you and let’s see what happens.