Have you run tcpdump to see what's provoking this? It seems like the router is perhaps configured for an MTU of 576 and is including this in an RA, which your system is trying to match. MTU discovery should just add routes to hosts (or maybe subnets) with a lower per-route MTU. Try ip6mode=none. That should prevent net.inet6.ip6.accept_rtadv from being set to 1, which should prevent an RA from being processed.
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