I will add into this that I have now had this on both a RPi3B+ and a VM install of Dietpi.
I followed some of the guidance and changed the DNS settings on the network settings in dietpi-config and removed the DNS entry for Pihole and this resolved the issue.
Restoring Pihole restored the issue.
I’m also using Pihole with Unbound. In Pihole, I changed the DNS from Unbound to Cloudlfare and it resolved the issue. Could the issue reside in Unbound?