They gave me a new address allocation. It didn't help. Then I noticed in an outbound traceroute that traffic was going via a random IP...it turns out another customer in the same subnet was sending out IPv6 Router Advertisements! I have now changed my default gateway configuration to ::1 from :: and things seem fixed. I hope the host goes after the offending customer and asks for them to quit it but I suspect not.

CC: @WiteWulf@cyberplace.social