Notification emails sometimes not sent

On several orders (about 10%) on my WooCommerce shop, the notification emails have not been sent.
I think this has to do with the status of the order due to the Sisow Plugin.
On the orders where the email have NOT been sent, the only order notes are 2x “Status ontvangen van Sisow: Success”, 2 seconds apart.
On the orders where the email HAVE been sent, the first order note is “Status ontvangen van Sisow: Success”, followed immediately by an order note “Order status changed from Pending to Processing”.
Is it possible that the order status change is skipped somehow? The status change from Pending to Processing is what causes the emails to be sent.