'watching first post' on a tag doesn't work on existing topic?

Just for curiosity/comparison sake, I’m testing the ‘watching first post in category’ behavior, and it’s not notifying upon recategorization.

My email time window mins is 10, & I’m leaving 15min between steps.
I’ve repeated this several times now…

  • Create new topic in un-watched sandbox category. Wait 15 min.
  • Move the topic into a category a user is Watching: triggers immediate notifications, sends email 10 min later.

  • Create another new topic in un-watched sandbox category. Wait 15 min.
  • Move the topic into a category where a user is ‘Watching First Post’: does nothing, ever.

(Self-hosted std. install, 3.4.0.beta2-dev (0c019b2e45), tests-passed.)

Maybe it’s something about my process, but I can’t imagine what. Is it possible the functionality that got sorted in 2018 has become unsorted?