MikroTik cannot be held guilty if it highlights improvised ISPs that don't know how to do their job...

Also, this is entirely fair. all my searching has definitely shown this is a comcast problem, and I am not trying to gripe about it as though it's MT's fault

under /system/logging open the warning entry (on most default configs it's #2 in the list) and add !radvd.

This does blow out ANY radvd warnings but at least my logs aren't burying important stuff in like an hour

This actually appears to be new behavior as of 7.9ish, see here

Not sure whether this is just new visibility into an existing problem, or what, but it sure does saturate my logs. I had to drop radvd warnings from being logged at all