Router advertisement frequency issue

I’m a residential Xfinity customer (gigabit service, Xfinity Gateway in bridge mode) in the Mountain region.

A 5-minute packet capture on my router’s WAN port shows IPv6 Router Advertisements arriving about every 1.7 seconds (171 in 300 s, median gap 1.68 s, max 3.81 s). I repeated the capture several times with similar results. They come from the router on Xfinity’s side of the line (the one that also answers as my IPv4 default gateway), not from my own equipment, and go to ff02::1. Router advertisements are normally sent every few minutes, so this is far more frequent than I’d expect.

It matters because my UniFi gateway pair (high availability) sends a sync update every ~3 seconds because of it, and Ubiquiti support has asked me to have Xfinity look at whether the interval can be reduced. I’ll try Xfinity support, but I’m usually on endless hold.

Two questions:

1. Has anyone seen this, and did anyone get it fixed or find a contact who can adjust it?
2. If you run your own router, could you check yours? tcpdump -ni <wan-interface> 'icmp6 and ip6[40]==134' shows the advertisements, and one every few minutes is normal.