IPv6 addresses cannot be manually configured through the UI
Trying to permanently work around this issue, I tried to manually configure the DNS server on my phone.
However, the UI seems to expect an IPv4 address, and it's impossible to type an IPv6 address into the field. Since my network has no native-IPv4 connectivity, this means that the manual DNS configuration is completely useless.
EDIT: This applies to all the fields in that UI, not just DNS server.