Not sure why thats the case for you. I'm on Vodafone/Kabel Deutschland too and got full ipv4/ipv6 dual stack? Is that one of those regional limitations that Vodafone seems to have?
What everyone seems to be missing here is that in France alot of heating during winter is done with electricity which is not the case in germany.
So peak electricity usage in France during Winter is much higher and that is the time where it needs to import alot.