Taking a closer look at the order_transaction_id field description for the Gift Card Adjustment resource, this field cannot be written manually. For the field to update, it requires an applicable order_transaction to occur that is associated with an adjustment, and will update automatically.
order_transaction_id A unique numeric identifier of the order transaction that issued the adjustment (if it was caused by an application of a gift card to an order).
I hope this helps clarify the expected results - Cheers!
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