XBOX network - why TEREDO if real IPv6 is present?

Maybe OT but i am interested ;-)

Running Windows10 64bit on a desktop PC suitable also for gaming.
Windows10 pushes the xbox services, game-bar etc. so i tried them out.

The PC is connected to a "FritzBox 7490" that is a device ADSL modem +
homenet-router (made by German company AVM)
This router-device in my case - although using IPv6 across a 6-in-4
tunnel-type, it does present to the PC
-- a fully qualified ipv4 address
-- a full IPv6 address + /96 subnet. IPv6 adresses from the subnet can
be properly reached from outside.

Now, when opening up the xbox live settings, it tells me fine that it is
connected, delay of 170 ms.
BUT in makes a TEREDO tunnel, instead of using my proper IPv6 address.
OK it works but somehow unnecessary and longer round-trip-delay.

Is there any way to tell the xbox connection-thing to use a real IPv6
instead of the teredo?

greetings

Roland Schweiger