How to publish IPv6 on duckdns

So for some reason, the way to publish an IP as recommended by DuckDNS themselves under the install-tab doesn't work. If you change the line doing curl from parsing echo through stdout and setting an url variable to just constructing the string and curling that, it works:

curl -s "https://www.duckdns.org/update?domains=$domain&token=$token&ip=&ipv6=$ipv6addr" -o ~/duckdns/duckdns.log

This version will let DuckDNS figure out your IPv4 and set your IPv6 according to what your device configured itself to as shown in the question. Make sure to run the script regularly as your global IPv6 may change if a NDP finds a conflicting address or if you haven't set a static IPv6.