What seem to be happening is that when WordPress avatar is set using Gravatar, changing the Gavatar doesn’t change the avatar URL, so the test for avatar_changed in discourse_single_sign_on.rb is failing.
I can change the wp_discourse plugin to send the parameter avatar_force_update = 'true' with the SSO parameters. That works, but it overrides the Discourse setting sso_overrides_avatar, so it will change user avatars when that setting isn’t selected.