I've got a few old notification in my events sidebar with the oldest being from early April, but recently (possibly starting today) I noticed that when hovering on an event to get the timestamp, it incorrectly shows the timestamp of my newest ping event.
So I got a forum ping 16 minutes ago, and when I hover on all my other events, including the one from early April as well as events in other sections like profile comments, they all say "16 minutes ago". Deleting the event from which the wrong timestamp is inherited does not return the other events to normal.
Edited to say that it may not be all new events that do this. I can't quite figure it out.
I hope that makes sense!
Firefox 89
This page may be helpful if you needed to know the specific time of your event :) I did notice this bug as well though, and this is just a workaround of sorts. Hopefully it's fixed soon!