Shopify randomly unchecking "Send shipping confirmation email" on orders

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About 1 in 8 times I fulfill an order, the box to send a shipping confirmation to the customer UNCHECKS itself for no reason I can see.

I don't normally notice (because it's supposed to be checked by default), and I get emails from clients wondering why their order hadn't shipped (even though it has).

Why would this happen?  Is there some situation in which it unchecks itself?  How do I fix this, it's causing me a lot of grief!