This morning, it seems that the Block Editor is really catching up with me.
I was able to compose the previous post using the Admin system, and it published as normal. However, I noticed a couple of typos, so clicked ‘Edit’ at the top of the post to correct them.
The Edit screen looked different, and was accompanied by a message that read “Welcome to the new Block Editor”. I made the corrections, and clicked ‘Update’ as normal. But then I could find no way of getting back to view the post. So I cancelled out of that screen, went back through the Admin page to view the post, and the corrections had appeared.
Now I am going to read my drafts more carefully, and try to correct any errors before publishing.