The message is <revreview-check-flag-p>. Presumably this doesn't get fetched in the new save dialog like it used to?

I see the same thing on Wikimedia sites, but cannot replicate the issue locally.

In FlaggablePageView::addReviewCheck, $this->article->isReviewable() returns false.

I made some uncommitted live hacks on deployment-apache01.eqiad.wmflabs and replayed the API request in Chrome until I got the right server. Turns out, $this->article represents the "API" title. Why is this not happening on my machine?

Change 127388 had a related patch set uploaded by Alex Monk:
Set context title when rendering edit notices and checkboxes

https://gerrit.wikimedia.org/r/127388

Change 127388 merged by jenkins-bot:
Set context title when rendering edit notices and checkboxes

https://gerrit.wikimedia.org/r/127388