Invalid MTU 9192 from xfinity for IPv6 (router advertisements?)

I'm trying to set up my IPv6 setup and I keep getting MTU packets of 9192 from Xfinity. Presumably these are router advertisements.

It seems my router learned the address for the gateway anyway, but it seems the upstream equipment might be misconfigured? The MTU of ethernet isn't 9192, so maybe this is the MTU that is supported over DOCSIS? I have my xfinity gateway configured in bridge mode to go directly to my Mikrotik router at home.