Undertow subsystem constructs the ProxyHandler instance with a hard-coded value (false) for the property reuseXForwarded:

https://github.com/wildfly/wildfly/blob/f34385abc92b7de3bbb63c46aca890e1ad39660e/undertow/src/main/java/org/wildfly/extension/undertow/handlers/ReverseProxyHandler.java#L162

It would be nice if we could configure it in the standalone.xml.