This is related to http://tracker.phpbb.com/browse/PHPBB3-10880
Noticed on phpBB.com. I am a member of the Website Team group, which has "never" set for the "can post without approval" permission in the Announcements Forum. From another group, I also have permission to approve posts in that forum.
Steps to reproduce:
1) Edit old post in the announcements forum
2) This now causes the post to become unapproved, due to PHPBB3-10880
3) Approve the post
This results in a new topic notification being sent to everyone watching that forum.