I've been in contact with Telenet today. They found an issue and fixed it.

Afterwards, the packet loss was indeed close to zero.

I've updated the post.
https://www.rivy.org/2016/12/ipv6-packet-loss-telenet/

The IPv6 test gives me 20/20.

And with regards to my home setup. I have an EMTA, so modem-only from Telenet with my own router. To exclude problems with the router, I connected a PC directly to the modem from telenet with a UTP cable. I still have the issue with the direct connection in both ...

I'm experiencing around 10% packet loss to any IPv6 address on the internet. 1 other Telenet user posted a comment to let me know that he experienced the same issue.

Is anyone else experiencing this?

More info : https://www.rivy.org/2016/12/ipv6-packet-loss-telenet/