ADMIN REST API POST /refund regression

[ilankessler]

Hi there, 

Since around 2025-05-20 15:30:00 utc we have been experiencing a regression in the /refund endpoint for some of our merchants.

We are now receiving the error: 

refund_line_items cannot refund more than refundable quantity

Things to note:

- The refundable quantity is available.

- It seems to be only an issue for retailers that have the new "Refund Method" option (on the new Customer accounts) available on their store. 

- I noticed this release was issued around the same time of the regression https://shopify.dev/changelog/apps-now-permitted-to-issue-refunds-to-store-credit