They’ve been playing again.
There are little brown boxes around some of the headings on my ‘New Post’ input screen.
Also on the categories menu list, sometimes no ticks appear when I select one, just another brown box so there is no indication that it will appear on the post as a category. Having just finished typing that, ticks are now appearing as I would expect.
I’m not going mad, honest?? Maybe it’s another experimental glitch for something that ain’t broke.