For force IPv6 or IPv4 you hav to use:

apt -o Acquire::ForceIPv6=true update

or

apt -o Acquire::ForceIPv4=true update

Ceate the file 99force-ipv6 in /etc/apt/apt.conf.d/ if you want it to be persistent.

#/etc/apt/apt.conf.d/99force-ipv6
Acquire::ForceIPv6 "true";