Adding some of our own most recent experiences on this:
We are closely tracking which orders the checkout_completed is and is not firing on (by evaluating Stape sGTM logs and comparing them to Shopify Orders), and realized that for the majority (but not all) orders that had Shop Pay or Apple Pay as the payment infrastructure, the checkout_completed never fired.
We have two separate pixels tracking this event, and neither of them receives it.
That said, I couldn’t reproduce this issue myself. Whenever I go and do a purchase through Shop Pay, I end up on the proper thank you page and I see the events fired and pixels picking them up.
Any info or clarifications from the Shopify side would be helpful here - we are losing a lot of events due to this.