Just mentioning this here as for a while I thought I’d messed things up.
The first post in a new topic Y was flagged as “off-topic” (which is an odd idea since it was the first post) and was automatically unlisted.
I moved it to an existing topic X (for separate reasons). It appeared there as unflagged.
Back to the topic Y to deal with the post. It was still flagged… Clicking “No, restore post” gave this error: “An error occurred: Body is too similar to what you recently posted”.
Presumably moving the post broke the link between the post in topic X and the post in topic Y.
Problem solved by clicking “Ignore flag and delete post”. (I think the flag won’t be remembered as being “not helpful” now but that’s not a big deal.)