I have confirmed that topos works correctly with this version, so the commit that broke things happened between 5.6.4 and 5.6.5
It might be related to how multi-homed installations interact with topos when doing double rr. Tne difference I noticed is that in 5.6.4, the topos substitution in Via is:
* inbound: Via: SIP/2.0/UDP 195.167.21.66:5060;branch=z9hG4bKq27z808ysq87yrzq6vp8m26yq;Role=3;Hpt=8e88_16
* outbound: Via: SIP/2.0/UDP 172.30.154.1;branch=z9hG4bKae0b.fce5449c22b5e79eb01d4f1ab3f7c014.0
That is kamailio replaces the hop with its internal interface. With topos on in 5.6.5, the behaviour is different for the UPDATE:
* inbound: Via: SIP/2.0/UDP 195.167.21.66:5060;branch=z9hG4bK1jo121j8j2h5h3wzh2mzj51q5;Role=3;Hpt=8ea8_16
* outbound: Via: SIP/2.0/UDP 185.73.42.241;branch=z9hG4bKc45b.e6226bcf14264ae3b8674dd25a71191c.0
185.73.42.241 is an interface on the same instance, but represents its "public" interface. Additionally, it happens to reside on a VRF, so the 200OK never reaches it because the routing table of the UAS directs it to another host with the same IP.
Hopefully this is enough to get an idea of what might have gone wrong? Thanks!