That example doesn't work, but an IPv6 address like: 3fff:0020::

Would be in the IP SAN as 3fff0020000000000000000000000000, which this code expands:

   "3fff0020000000000000000000000000"
                .toLowerCase()
                .match(/.{1,4}/g)
                .join(":")
                .replace(/\b:?(?:0+:?){2,}/, "::")
   '3fff::20:0000:0000:0000:0000:0000:0000'

Which has one too many parts and doesn't parse as an IPv6 address. But like mentioned this is just presentation code. I don't want to waste time if this isn't actually a bug, but maybe someone on the LetsEncrypt trial could actually make a cert to see if IP addresses formatted like that are a problem in reality...