Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
It seems like that is current default behaviour, as long as you already published that blog post then you unable to switch those blog author through that draft post, unless you update that draft post to live.
By the way, I will consult with our developer regarding this on this coming Monday and see whether supposed to work this way.
Regarding with your SEO keyword issue, when you fill in that keyword, you have to press enter key from your keyboard so it only can store that correct keyword into your blog post.