If you think it would help, I can provide a copy of the hr plugin for the Atto editor that I have working in Moodle 3.1.2+. But, as mentioned, it is nearly impossible to see, and therefore, I always wind up editing manually to get a line I can see.
Hmm, it just occurred to me that this would probably be another place to use a Generico filter with height, width, and color settings. I'll add that to my, to do, list of things to try.