Hi Arnold-
Thanks for reaching out, and thanks for using the Post and Page Builder for WordPress.
I was able to replicate the same problem you’re having, and it looks like a small bug. I’ve filed a bug report for our developers to fix this in a future release, but here’s how you can fix it right now.
When you click on the <hr> element, switch over to the text editor and add the class bg-hr to the element, and then the control will work. Check out this brief animation where I demonstrate: