Checkout response does not return shopify_payments_account_id

[sendosoEng]

Good morning. I am a developer for a wider company and on our Shopify account we have different stores. On one of those I am testing the Checkout API to get a sales channel with payments. It's all good except the payments bit. According to this guide

https://shopify.dev/apps/channels/complete-payment#complete-a-payment-using-stripe

"When you create a checkout, Shopify will return the merchant's shopify_payments_account_id."

Which is returned, but it is always null.

I have looked for similar problems on these pages and found many others, but none with a clear solution. Can anyone help please?