I am logged in as the site admin in Chrome on OS X. I have four empty categories in a new forum, and only two categories with a post in them.
Occasionally, when entering the categories page through the menu to the top, I see only the categories with posts in them:
When refreshing the categories page, I get the full list, which seems to be the desired behaviour now:
So it seems like the markup served on a normal request differs from that fetched through Ajax. However, this is not stable behaviour! After a while, it will go back to correctly showing all categories even when I go back and forth in the menu.
However! The phenomenon seemed to start when I opened a separate, anonymous Safari session. There, I see only the two categories with posts in them, which I suppose is deliberate and OK because I’m not logged in. Now whenever the broken behaviour vanishes in my admin session in Chrome, I just have to switch over to the anonymous Safari session, and visit the categories page again (which will show the two categories.) When I then switch back to Chrome, it will start showing the broken behaviour there!
The only thing that could cause this is a caching issue, I think. I have Redis installed if that helps.