That is too bad…
If you want to get to the bottom of it and invest a bit time/effort you could run a tcpdump on the TO and analyse with a tool like wireshark the packet traffic between the FB (when rebooting/cold started) and the TO.
It should reveal whether the TO is requesting the ipv6 from upstream and also whether the FB is broadcasting it to the TO, respectively whether the packages are reaching the TO.
To my knowledge AVM does not provision tcpdump in their proprietary OS for the FB.
If I am not mistaken the tcpdump packet capture should cover traffic on UDP ports 546 and 547, e.g. on the TO via ssh cli
tcpdump -i [put the TO wan6 iface name here] udp portrange 546-547 -XXevvvw /tmp/[name your dump file here]
Once the capture is done download the dump file from /tmp via ssh to a client that runs wireshark and import the dump file for inspection