Here's my original post on Meta Italian.SE, which I'm now transposing here since I'm now positive that this is happening network-wide.
I've noticed that edits of other users performed on my own posts are not notified on some sites across the network (they are notified e.g. on Ask Ubuntu but they're not notified e.g. on Italian.SE and on Meta.SE), despite this happening on other sites across the network.
The last time this happened was just a few minutes ago:
As you can see the latest notification hails from 29 minutes before the screenshot was taken, and there's no notification of the more recent edit whatsoever.