I had report this bug by email. But I didn't got a response.
I am using Alfred 3 now.
My mac has a private ipv4 address (e.g. 10.0.0.1) and a public ipv6 address (e.g. 2001:db8::1).
Now I can connect Alfred from my iOS device using ipv4 address.
But I can’t use ipv6 address to connect to Alfred. (I tried using a domain name. I tried setting up a direct connection within Alfred.)
I notice Alfred only listen to the ipv4 address.
-------------update------------
It not a bug.