I don’t know how to patch the plugin to make it work, but in my case I wrote my own validation function to check the characters in the username, then (knowing I had a valid username) just unset the error returned using the wpmu_validate_user_signup hook.
https://codex.www.ads-software.com/Function_Reference/wpmu_validate_user_signup
function twig_filter_wpmu_validate_user_signup( $userinfo ) {
if ( array_key_exists( 'errors', $userinfo ) ) {
unset( $userinfo['errors'] );
};
return $userinfo;
}
add_filter( 'wpmu_validate_user_signup', 'twig_filter_wpmu_validate_user_signup', 999 );
If you are going to use my code snippet, just be sure to validate the username first, otherwise ANY username will be allowed, and that could cause problems down the line with WP core logic ??