When a request is sent to APIcast including the Range header, the header is not forwarded to upstream: it appears APIcast handles the response range before returning the response to the client.
However, when the response has Transfer-Encoding: chunked, (and therefore no content-length), the Range header seems to have no effect.

See the attachment upstream_apicast_req_resp.txtfor an example of APIcast behaving as expected.
See the attachment upstream_apicast_req_resp_chunked.txtfor an example of APIcast not behaving as expected when the response from upstream is chunked.