Did anyone manage to find a solution to this using ACF?
I have tried to change the priority of the order completed email from Woocommerce but it still fires before the ACF field updates.
Here is what I tried:
add_action( 'woocommerce_email', 'unhook_those_pesky_emails' );
function unhook_those_pesky_emails( $email_class ) {
remove_action( 'woocommerce_order_status_completed_notification', array( $email_class->emails['WC_Email_Customer_Completed_Order'], 'trigger' ) );
add_action( 'woocommerce_order_status_completed_notification', array( $email_class->emails['WC_Email_Customer_Completed_Order'], 'trigger' ), 90, 2 );
}