ADF copy activity for REST API POST requiring string value in request body field

I have a ADF copy activity where my source is a REST API POST statement to a report process in one of our data sources. I have the parameters for the report stored in a SQL server table and I am calling it from a pipeline parameter. My expression for the request body field is this:

@json(pipeline().parameters.source_filter)

When the pipeline runs, the request body values look like this which look correct.

"requestBody": { "Name": "Solidigm FG Product Data Report-CSV", "ContainerPath": "/wt.inf.container.OrgContainer=Solidigm", "Criteria": []

But I am getting the following error:

Failure happened on 'Source' side.ErrorCode=UserErrorInvalidValueInPayload,'Type=Microsoft.DataTransfer.Common.Shared.HybridDeliveryException,Message=Failedto convert the value in 'requestBody' property to 'System.String'type. Please make sure the payload structure and value arecorrect.,Source=Microsoft.DataTransfer.DataContracts,''Type=System.InvalidCastException,Message=Objectmust implement IConvertible.,Source=mscorlib,'

Per searches on the web, there is a suggestion for adding the REST header of this:

content-type: application/json

I already have this and still not working. I believe this is a bug with the copy statement.

any help is appreciated.

I have search through all internet searches and found nothing.