Temporary failure in name resolution - only on localhost

[Bucking_Horn]

That's expected behaviour since 5.0:

@Pi-hole v5.0 is here!

• Pi-hole no longer changes the nameserver assigned to the Pi (in /etc/resolv.conf). The existing nameserver settings for the Pi-hole host are respected and unchanged.

See also Correct Resolv.conf entry in pihole 5.11 - #2 by jfb, where jfb explains the ins and outs of picking a DNS server for the machine that hosts Pi-hole.