I can do ping6 from interface address to outside world.
I've set floating rules to block ipv6 inbound traffic to local interfaces and to allow all outbound traffic.
all is set as per manual in opnsense docs
After having manually added to aforementioned 4th client (debian server) gateway (gateway’s link-local) all works fine. So, question is why most of clients in other vlans don’t receive default gateway address or don’t configure their interfaces upon receiving it?