The release notes say "Resources will now default to opening on the right, while guides will open on the left; when opened in a blank layout.".
If I have a blank layout, then open a guide, the guide opens on the left, as promised. Subsequently all guides open on the left, all resources open on the right. That works as advertised.
If I have a blank layout, then open a resource, the resource opens on the right, as promised. But all subsequent resources (and guides) open on the left. That doesn't work as advertised.