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https://opensearch.org/docs/latest/api-reference/document-apis/update-by-query/

The documentation on this page says that _source param will change the data in the results from the operation, however while testing in v2.17 Dashboard, it looks like any fields excluded from source during an update by query operation (and not set by the script) are not written to the final results - effectively erasing all excluded fields from the document.

Probably a copy/paste error, as this is a significant departure from documentation.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
Tested in v2.17, same error present in v2.19 docs and likely most/all previous

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
Create documents, populate with 3 fields\values. Update by query to change the value of 2 of those fields, in _source exclude the third field from this operation. Query the docs afterwards, the third field is gone.