Really a mistery to me. It’s a multisite installation but I do not think this is the issue. Just one plugin, your Premium pack (tried to disable, no success). By the way, same problem if I use GP original and not a child theme.
I dequeued your mobile.css, moved it to my-child-theme/css, re-enqued thw file, no change, always code 301. As I said all other css files in the same path are loaded no problem.
I gave up for now, I dequeued mobile.css and put all its content into the child theme style.css.