The problem:

Minecraft disables IPv6 the moment the launcher runs the game.

The cause:

Incorrect use of initialization flags by the launcher when running the game in any version: -Djava.net.preferIPv4Stack=true

How to solve:

Change the flag value and add another one:

-Djava.net.preferIPv4Stack=false -Djava.net.preferIPv6Addresses=true

Making the game work with both protocols simultaneously.

This issue affects all versions of Minecraft Java(From 1.8 to recent versions.) due to an incorrect launch command. There are several reports of this problem and it was never fixed.