PPPoE (mpd5) IPv6 issues

Hi freebsd-net!

 

After a lot of google and tinkering I have hit a brick wall trying to set-up
my new ISP which uses PPPoE.

I'm at the point where IPv4 works fine 100% for all websites. But from the
moment I turn on IPv6 all sites that prefer IPv6 stop working, I have
confirmed that routing table etc all look good. So I'm more suspicious to a
MTU / MSS issue.

Although I cant find that much info about it. I have tcpmss fix turned on in
my mpd.conf

 

There seems to be a few sites like google.com and youtube.com that keep
working.

 

I have googled thoroughly but I was unable to find a fix for my problem. The
one close to my issue seems  this issue from 2020: