I've had this issue for a few weeks and I'm still not close to figuring out what's wrong. Pages I've used the displaytitle magic word still have the old title in the header. The names change just fine on links so I know it's active.
I cannot delete and just make a page with a different name because old titles are numbered to make it easier for admins.
I use only extensions provided by miraheze so everything should be up to date, if it's an extension that is causing the problem I can't figure out which it is. I ruled out HideTitle and SkinsPerPage.
Before anyone asks: Restrict Display Title is set to false, Allow Display Title is set to true.