Hi @Valentin Duhamel ,
Thank you for posting query in Microsoft Q&A Platform.
When we use REST connector inside Copy activity and make APIs calls with pagination rules. Then each time whatever the response we get from API it will simply get appended. We cannot control the responses to get merged in previous response at some particular node.
Hence you see data that way getting appended.
We need to either process this entire JSON after copy activity as per our needs using dataflows or to make API call and foreach on items and merge them into one.
Hope this helps. Please let me know how it goes.