Saving user meta in custom table not working

I'm trying to set up custom table for saving custom user meta, but I cannot get it to work. It works fine if I select post for custom fields group type, but not if I select user.

Here's the code I'm using for table creation:

add_action( 'init', 'prefix_create_user_table' );
function prefix_create_user_table() {
    if ( ! class_exists( 'MB_Custom_Table_API' ) ) {
        return;
    }
    MB_Custom_Table_API::create( 'prefix_users_data', array(
        'user_credits'   => 'BIGINT(10)',
    ));
}

For custom fields group definitions I'm using Metabox AIO and builder, with all extensions enabled.

Where's the catch?