I expected that min first post length would affect only the first post in a topic and min post length would affect all the replies.
Instead, min first post length affects the first post and any replies to that post.
This doesn’t make sense to me. The point of a longer minimum for the opening post is that you want to make sure the topic is stated clearly enough. I don’t think there’s any reason that replies should have a different minimum depending on whether they’re replying to the original post or to a reply.