Is there any reason why this is now happening?
There haven't been any changes to the site in that area in several months.
When you say months, is there any reason why the site is now working like this?
My annoyance is that I had a couple of sessions open on my computer and several (20+) panels were opened in individual tabs. Then my browser crashed and when I re-opened it, all of the pages showed a not found banner.
I was able to reproduce this reliably and determined that my browser restart following a crash wasn't keeping the referer header, but I don't really understand the reasoning for requiring the referer header. Relying upon it offers zero protection or security beyond making users miserable.