Current implementation of proxy.config.http.use_client_target_addr opens a very simple attack vector for cache poisoning in transparent forwarding mode.
An attacker (or malware installed on innocent end-user computer) puts a fake IP for popular website like www.google.com or www.facebook.com in hosts file on PC behind the proxy. Once an infected PC requests the webpage in question, a cacheable fake response poisons the cache.
In short, while requesting an URL from origin server IP as hinted by the client, proxy makes independent DNS query in parallel in order to determine if client supplied IP belongs to requested domain name. In case of discrepancy between DNS and client IP, the transaction shall be flagged as non-cacheable to avoid possible cache poisoning, while still serving the origin response to the client.