hi @AndyS I had it now for the 2nd time that this happened to me.
Once is chance, twice may be something to investigate
Take look at this screen shot:
IMG_0068.PNG
It is right after a page brake (20 posts in my settings). Right after the first new post on this page, appears an older one from the previous page
IIRC, than I had this twice now, on different devices, (Macbook and iPad)