There is a fairly robust and tested library here for validating ipv4 and ipv6 addresses:

https://github.com/moodle/moodle/blob/main/lib/classes/ip_utils.php

but the admin_setting_configiplist predates this and hand rolls its own validation using a regex which is not working:

https://github.com/moodle/moodle/blob/main/lib/adminlib.php#L4229-L4232

Reproduction steps:

1) Visit /admin/settings.php?section=ipblocker

2) Enter any valid ip6v address eg 2001:db8:3333:4444:5555:6666:7777:8888 and save

3) This produces a validation error