OK, so I found the problem…
I have a process that marks order->fulfillments->state = ‘COMPLETED’ each night for orders that weren’t marked manually via the POS, but I wasn’t setting order->state = ‘COMPLETED’. Up until a few weeks ago, this didn’t matter. Now, I guess it does. Obviously, I fixed it on my end…
But, I think it’s really a bug on your end: an order with a state != '‘COMPLETED’, but with order->fulfillments->state == ‘COMPLETED’ cannot be deleted via the POS.