I think it is important to keep the original issue in mind. Watching First Post is a released feature that should work for tags.
I don’t have reproduction steps. But maybe a moderator can split this discussion to better keep track of the bug portion of it.
I don’t think there’s sufficient evidence here to suggest that it isn’t working.
One of the team could check in your specific case as to whether email was sent to your account, as it stands though you’re the only person reporting a problem.
Thanks for posting, this happened to me as well.
I am watching the release-notes tag (not just the first post) for quite some time. And the 2.8.5 Security Release was the first and only one I got notified about.
It does not seem like an email issue to me, because my notifications here on the site also only show the 2.8.5 release post.
I’ve been keeping an eye on this to see if it happened again, and it appears you didn’t receive a notification from the latest 2.9.0.beta6: Whisper Improvements, Loading Splash Screen, Security Fix, and more release-notes topic. I am going to split this out into a bug topic as you suggested. 
The tag notification was not created if the topic was created from a shared draft. I fixed the issue in this PR:
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