Users Select inside custom metabox

I think the problem is you are passing the WP_User_Query object, not the results of the query. Try to change:

$fields = array(
array( 
        'id'   => $prefix . 'user_sub', 
        'name' => 'Subscriber User', 
        'type'     => 'select',
        'use_ajax' => false,
        'options'  => $user_query,  // this is where you populate the select in metabox
    ),
);

To:

$users_ids = array();
if ( !empty( $user_query->results )){
    foreach($user_query->results as $user){
        $users_ids[] = $user->ID;
    }
}

$fields = array(
array( 
        'id'   => $prefix . 'user_sub', 
        'name' => 'Subscriber User', 
        'type'     => 'select',
        'use_ajax' => false,
        'options'  => $users_ids,  // this is where you populate the select in metabox
    ),
);