The 3.1.4 ATS build - trafficserver is broken from 3.0.2 and does not listen on the IPv6 interface when server_port_attr is set to =6

On ATS 3.1.4 changing server_port_attr to:

CONFIG proxy.config.http.server_port_attr STRING =6

...doesn't bring up an IPv6 socket? We get:

1. netstat -an | grep 3128
   tcp 0 0 0.0.0.0:3128 0.0.0.0:* LISTEN

...previously on ATS/3.0.2 this was fine:

1. netstat -an | grep 3128
   tcp 0 0 :::3128 :::* LISTEN

This will have a major impact on IPv6 functionality.

Any ideas what's wrong or how to fix this - or workaround this?