Missing source_order_id or source_order_transaction_id in Shopify Payments Transaction API

Shopify Partner
81 1 11

Hi there,

In the Shopify Payments Transaction API, I have seen situation where fee is a positive amount but source_order_id or source_order_transaction_id is null (some time both are null)?

Why is there a fee when it is not tie to an order or transaction? 

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