If you want to mirror all the sip traffic, just set this modparam:
* https://www.kamailio.org/docs/modules/stable/modules/siptrace.html#siptrace....
If you still want to do selective mirroring, for outgoing ACK you have to use onsend_route block, because the ACK doesn't have a corresponding response, there cannot be a transaction for it, so it is forwarded in stateless mode.
Should this still not work, then reopen.