Fixed… at least for me
By simply unplugging my iPhone from my Mac and then running ionic serve again fixed the problem.
It appears that ionic serve listens on all network interfaces which was causing a problem when my iPhone was plugged into my Mac. Mac uses the en5 interface to access the internet when tethered to your iPhone via a USB cable).
ifconfig from bash and look for interface en5. Specifically the ‘broadcast’ address.
en5: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::1cd2:9644:4838:322%en5 prefixlen 64 secured scopeid 0xd
inet 169.254.116.231 netmask 0xffff0000 broadcast 169.254.255.255
media: autoselect (100baseTX <full-duplex>)
Hope this helps.