proxy.config.http.server_session_sharing.match has a default value of "both", which compares IP address, port, and FQDN when determining whether a connection can be reused for further user agent requests.
The "host" (FQDN) matching does not behave safely when ATS is operating as a reverse proxy. The compared value is the origin server FQDN after mapping, rather than the initial "Host" target.
If multiple Hosts map to the same origin server and the scheme is HTTPS, ATS will attempt to reuse a connection that may have an SNI Host that does not match the HTTP Host. With Apache 2.4 origin servers this results in 400 Bad Request to the user agent.
You can observe this behavior with two mapping rules such as:
Non-caching clients alternately fetching URIs from the two targets will see 400 Bad Request responses intermittently.
proxy.config.http.server_session_sharing.match should have a default value of "none" when proxy.config.reverse_proxy.enabled is "1"
In order of completeness:
1) Do not share server sessions on reverse_proxy requests.
2) Do not share server sessions on reverse_proxy requests where scheme is HTTPS.
3) Compare target host (SNI host) rather than replacement host when determining if reuse of server session is allowed (when server_session_sharing.match is set to "host" or "both").