On regular topics, editing someone else’s post generates an ‘edited’ notification (notification_type: 4).
However, if the topic is a PM, the edited pencil appears (because a post_revision is generated) but the recipient is not notified and no notification appears in /notifications/edits.
Along the same lines, I think some weeks ago, when editing someone else’s PM, the button would display “Save edit”, but after a recent update it now displays “Message”, which is confusing since the message has already been sent.
The only other alternative I can think of is that there’s a problem with the translation (my site is in Spanish).