Order by custom field value not working for acf date field

There seems to be a bug in WordPress building the query. I had this exact same issue today, using ACF Pro. I passed an array to orderby, and it worked. This is my code:

$args = [
        'post_type'         => 'event',
        'meta_query'        => array(
                    'key'       => 'event_date',
                    'value'     => date('Ymd'),
                    'type'      => 'DATE',
                    'compare'   => '>='
        ),
        'meta_key'  => 'event_date',
        'orderby' => array( 'meta_value' => 'ASC' ), 
    ];