Help needed in adding field to existing index

I am using OpenSearch locally, I have indexed some documents along with knn vector field, I needed to add an additional field to the existing index so, I tried according to the official documentation for update documents.

But when I update an additional field all other field gets collapses mean I cannot search on them it returns empty value. Is there any way to add an field to existing index, obviously I can reindex but I can’t do that often in production right. So, please let me know a solution for this .

I have also set dynamic mapping as true. I am using 2.15 version

Could you show how you use the update document API?