It looks like kamailio fails to parse them and correctly route for example an ACK to a 200 OK.
Should have looked better....
It's not the Route header causing the issue.
Cisco-SPA112 with most recent Firmware (EOL, no more bugfixes). Fails to parse IPv6 Addresses in the Contact Header and considers the number between the the first two colon, to be the Port, ignoring the remaining part of the IP address, completely garbling the RURI of the ACKS.
*sigh* I guess we will have to tell our customers, that those devices can not be used anymore when we allow any customer to communicate via SIP IPv6.