Dogtrousers tagged me today and I did not get an alert. It appears that if a post is created without a tag, and later edited to include a tag, the edited post does not generate tag alerts when saved.
This may be by design to stop multiple alerts being generated if a post with tags is repeatedly edited and saved.
I think each post should have a list of tags that have generated alerts. When an edited post is saved, any new tags in the post should generate alerts, and then be added to the list so they don't on subsequent reposts.