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' ),
];