No idea if this has any relevance for your problem, but the discussion in the following thread gives two alternative explanations: losing ipv6 connectivity due to ipv6 address timeout at 30 minutes or some multicast problem (no idea what that actually means...).
It is quite possible that as offloading bypasses normal routing to some extent, some trigger that would keep the connection alive, temporary ipv6 address renewed, or something gets also bypassed.