Same for me. I'm running v6.0.6/v6.1 and am seeing a DNS resolution failure every hour, coincident with the following message appearing in the FTL log:
2025-04-09 17:01:07.286 ERROR Cannot receive UDP DNS reply: Timeout - no response from upstream DNS server
2025-04-09 17:01:09.838 INFO Tried to resolve PTR "105.0.168.192.in-addr.arpa" on 127.0.0.1#53 (UDP)
It looks like pihole hasn't got a PTR record for a host (192.168.0.105) it's assigned an IP address to via DHCP.
Anyone got any ideas what's going on? I've recently upgraded to v6 and wasn't seeing this issue in v5.