Solved my own problem.
I forgot this needed to be within a Post/Page Loop, as well as my prefix wasn't rendering out as "fbm" it was being stored as code "{$prefix}_sidebar_group"
What was the result of this? I have the same problem, Willing to talk here about it so others can learn and resolve their problem.
I can't even get this to return anything
$group_values = rwmb_meta( 'sidebar_group' ); var_dump($group_values);
