OP = Only Post or Opening Post?
I tried in our Discourse and if i checked it right, Notifications work only if the wiki is the only post in the Thread. In threads where the wiki is the opening Post, but not the only posting, notifications are not working.
Edit: Checked again, with 2 new Test Postings. Notification is Working on both of them, even if one has several answers. Anyways we have one Wiki in our Discourse which doesn´t behave like that - no notifications for people who follow.
Is This behaviour different when the post is a sticky post?