If you can figure out which theme, plugin, or other CSS is causing it, someone might be able to write a snippet to fix it. Or you might consider not using that add-on.
I specially turned of all 3d party plugins and themes to find it out.
And after it in default theme I get the same result (see screenshot). Just remind you I’m using Android, and my routine theme is “Minimal”. But in defaul one there’s the same issue.
The same as @dawni, I don’t see any issues on my end using your original snippet.
That said, hr alone isn’t that specific. Adding (for example) border-color: red to the original, you can see changes in the UI (color and margins) where you probably don’t want them.