On myxwiki.org I've noticed that whenever I was going to my subwiki and clicking the bell icon, the notifications area kept spinning forever. I actually only waited 3 minutes before stopping.
see the attached video to visualize what I experiment there.
Note that myxwiki.org has a lot of subwikis and it's using the default config for the activitystream storage.
On the main DB (xwiki):
Funily if I do the same query on my subwiki DB (i.e. massol) it's not empty: