This is not a issue, but discussion.

lhttpc_client.erl uses {Host, Port, Ssl} tuple for lookup keep-alive sockets in lhttpc_manager.erl
So:

1. client connects to server {Host, Port, Ssl}
2. server returns response and keep connection alive
3. lhttpc keep open socket on lhttpc_manager under {Host, Port, Ssl} key
4. client makes one more request to that host {Host, Port, Ssl} and can reuse this socket

But in case when proxy enabled, there is one more component that involved in the process:

1. client connects to sever {Host, Port, Ssl} through proxy-server {PHost, PPort, PSsl}, by specifying proxy option
2. server returns response and may keep connection alive; proxy keep connection with client alive
3. lhttpc keep open socket (connected to proxy-server {PHost, PPort, PSsl}) under key {Host, Port, Ssl} !!!
4. client makes one more connection to that host {Host, Port, Ssl}, but provide OTHER proxy option. But lhttpc_manager return cached connection to proxy-server {PHost, PPort, PSsl}

My suggestion: include proxy option in the cache key, eg host {Host, Port, Ssl, ProxyUrl}