That was happening on my dad’s WordPress. Someone input a whole bunch of spam comments without reference to a post. Then when he posted the comments would attach to the posts one at a time. All the time the comments were closed. It is an older version of WordPress. I had to go to to “Edit” and start deleting them. There were so many of them I went to mysql and deleted them at once. It’s as if the spammers learned how to queue up the spams waiting for posts.