only listen "0.0.0.0" on IPv6 Only environment

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My environment only has an IPv6 address and does not have an IPv4 address.

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 10:70:fd:6c:2b:34 brd ff:ff:ff:ff:ff:ff
    inet6 fdbd:dc02:11:924::20/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::1270:fdff:fe6c:2b34/64 scope link 
       valid_lft forever preferred_lft forever
3: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 10:70:fd:6c:2b:37 brd ff:ff:ff:ff:ff:ff
    inet6 fdbd:dc02:11:925::20/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::1270:fdff:fe6c:2b37/64 scope link 
       valid_lft forever preferred_lft forever

When I use uvicorn.run("app:app", host="") to start the service, uvicorn only listens on the address 0.0.0.0:8000, and it does not listen on the IPv6 address. Why is that?

uvicorn: 0.22.0