When selecting "Open to LAN" from the game menu, the game only listens over IPv4. On pure-IPv6 hosts, this means that minecraft basically only listens for localhost.

Listening to ipv6 be default should be desirable, since most OSs will actually also listen on ipv4 if you listen to ipv6, meaning you get to keep backwards compatibility at no cost (and you don't actually have to open two sockets, etc).