In our case, we have a network with several subnets, each one with a DNS set. So I believe this would line up with what your operator has reported.
I've also noticed some other odd behaviour from the L3 agent where it seems to have a high loss and latency when handling udp traffic periodically. This started happening around the same time as the radvd issues, but I haven't fully narrowed down the cause yet.
In our case, we have a network with several subnets, each one with a DNS set. So I believe this would line up with what your operator has reported.
I've also noticed some other odd behaviour from the L3 agent where it seems to have a high loss and latency when handling udp traffic periodically. This started happening around the same time as the radvd issues, but I haven't fully narrowed down the cause yet.