When a request is made by WebClient to a service that returns several headers of the same name but with different cases, e.g.:
"Set-Cookie" with value "aaa"
"set-cookie" with value "bbb"
"SET-COOKIE" with value "ccc"
then the response object contains only one header value, all other values are lost.
The issue is not reproducible in all cases, but only with some service providers and is probably caused by low-level details of provider implementation.
In the attachment there is a SoapUI REST mock service and CXF WebClient-based consumer with which the issue is reproducible.