I don’t think you want get_stylesheet_uri, which will return the complete stylesheet URL including style.css. You want get_stylesheet_directory_uri, which will give you the path up to the child/parent theme stylesheet directory but not the trailing style.css, and which is the function used by the Roots theme per the code you posted.
wp_enqueue_style('fsc', get_stylesheet_directory_uri().'/abcdefg.css', false, null);
I also notice that you didn’t provide the actual stylesheet name so I tacked on /abcdefg.css.
It is interesting that the URLs are not absolute URLs. That could be a plugin, a theme, or some server module like Google’s PageSpeed, which will strip the domain like that.
The work-around !
Per a chat with Lord_Mort, we worked out that the Roots theme appears to be rewriting requests to either /themes/ or /wp-content/themes/ which makes it difficult to add stylesheets normally. Lord_Mort discovered that a path like wp-content/beans/fsc/style.css is not rewritten, so the workaround we came to is to is to use the same trick that Roots uses and add stylesheets to a non-standard location that will not be rewritten. Lord_Mort’s decision was to use the same one Roots uses.
wp_enqueue_style('fsc', get_stylesheet_directory_uri().'/assets/css/style.css', false, null);
Without installing that theme I don’t think I can debug and solve the issue more elegantly.