For the metafield, there are many metafield that you can apply. And there also provided a few settings as well. So, when you choose the metafield type in Stock Sync must be the same as Shopify.
When you use this https://big.com/sneakers/hello-3-main-dark-iris-cl8532-105 in the metafield. You should choose the correct metafield type.
If you used metafield type like string or single-line text it will get an error. You can see the error in the Changes log file.
The error will look like this "Type 'single_line_text_field' must be consistent with the definition's type: 'URL'. " in the error remark column.
So, you should choose the correct metafield type. And its also must be the same as Stock Sync and Shopify.