Hello Bens Bernard,
Welcome to the Microsoft Q&A forum.
From your issue, it seems like auto mapping attempts to map source and sink fields by names and data types, but if the names don't match or if you're trying to change the field names, it may fail because it can't find a corresponding field in the destination.
When you use dynamic content with @json(myjsonmapping), you're providing a JSON string that explicitly defines the mapping, which allows for more flexibility and control over the field names and how they're mapped. This can be particularly useful when the source and sink schema do not match or when you need to transform the data as it moves from source to sink.
Please see the below reference documents:
https://learn.microsoft.com/en-us/azure/data-factory/copy-activity-schema-and-type-mapping
I hope this helps. Please let me know if you have any further questions.