Unset not working in pipeline

With source connector 1.7.0, I do the following:

"copy.existing.pipeline": "[{\"$unset\": \"Payload\"}]",
"pipeline": "[{\"$unset\": \"Payload\"}]"

The pipeline for the copy-existing stage (copy.existing.pipeline) works as expected, and the Payload field is excluded but for the change stream events after the copy-existing stage, the “pipeline” is not working although it’s exactly the same as the “copy.existing.pipeline”.
No errors in the kafka connect log.

What could be the reason for that?