User blog comment:Rappy 4187/Technical Update: September 18, 2012/@comment-2156322-20120924171908/@comment-188432-20120924175557

It's not that the categories aren't there after the first edit. It's merely that they don't display. Yes, this is a bug, yes it should be repaired, and yes it's only recently started happening — but it's best resolved by reloading the page, not by editing a second time. If you edit a second time, you're probably going to end up putting the categories on for a second time.