Thanks @thoeg @nolan, much appreciated. I think I see what the issue is. We’re currently using an empty span element to set the focus to the post. This element has both aria-hidden=true and tabindex=-1, and I think that makes the element invisible to screen readers.
I think it may be best to switch focus to the first focusable element in the post. In most cases that will be the username link, i.e. the post author.