Thank you cfladger for the detailed explanation. I followed your steps to modify GRUB and then found an article on (Linuxconfig.org) to make the change in sysctl.conf (https://linuxconfig.org/how-to-disable-ipv6-address-on-ubuntu-18-04-bionic-beaver-linux). It seems that with version 18.04.1 the bug is fixed so that when I restarted after modifying sysctl.conf, IPv6 is disabled.
I hope this helps others.
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