I think it's pretty obvious where the problem is ....
https://test-ipv6.com/ works great:
Screenshot 2022-12-18 154317.jpg 84.83KB 0 downloads
https://ipv6-test.com/ is having issues:
Screenshot 2022-12-18 154652.jpg 71.52KB 0 downloads
And I'm pretty sure they used the OVH Roubaix node in the past ...
.. which should be this node with 70% and more loss:
6. AS16276 be102.rbx-g3-nc5.fr.eu (2001:41d0::25f3) 0.3 1.5 4.5 70.0% 50 15 19.9 21.2 18.6 26.4 2.4