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Помогло следующее.
В /wp-includes/class.wp-scripts.php
к коду
$tag = "{$cond_before}{$before_handle}<script type='text/javascript' src='$src'></script>\n{$after_handle}{$cond_after}";
добавил
async $tag = "{$cond_before}{$before_handle}<script type='text/javascript' async src='$src'></script>\n{$after_handle}{$cond_after}";
нашел на moneyroute