Original Content-Length header was removed as its value was invalidated while using APIM XML to JSON conversion policy.

This typically happens when APIM does the conversion from XML to JSON as the content length of the original XML differs from the resultant JSON, thereby invalidating the original Content-Length header. Use the xml-to-json policy in APIM for conversion, setting attributes like kind, apply, and consider-accept-header appropriately for your needs. If dealing with request or response body issues, consider utilizing the set-body policy to modify the content accordingly. Remember, the placement and configuration of these policies are crucial for proper application.

learn.microsoft.com/en-us/azure/api-management/xml-to-json-policy

learn.microsoft.com/en-us/azure/api-management/set-body-policy