My sister’s WP blog had a similar problem. There are three admin users. She was writing the post using her admin user. But when she tried to update the Post Author field, she could only select my admin user or the other admin user.
I was able to force her to be the author by temporarily changing her user to be an editor, and then updating the post author for her. Then, when I set her user back to an admin she stayed the author. Very strange…
