HotCat was recently changed (T103285) to listen to wikipage.content, but on the upload page, we trigger mw.hook wikipage.content for the preview of the file, so that explains the behavior. HotCat is now listening to the wrong event.

@TheDJ: When I had HotCat listening for the postEdit hook, you said that wasn't the right hook to use. I then changed it to use the wikipage.content hook instead, which you say is also the wrong hook. If you know which hook HotCat should be listening for, can you please just tell us?

@kaldari wrong is a big word.

But anyway: wikipage.categories was introduced almost at the same time, and probably counts as the 'right hook' https://gerrit.wikimedia.org/r/#/c/219629/

But that one probably doesn't even work yet icw VE. And as this ticket testifies, at that time wikipage.content was not even triggered on the upload page, so it's all a bit of baby step forward causing a sidestep once in a while.