I'm running pi-hole as a docker container on a Windows 10 PC. I've had this happen twice now (that I've noticed)... my remote PowerShell session to Exchange Online dies (which is when I notice it) and in looking at the Upstream DNS servers, Cloudflare is selected and the loopback address is specified as well. When setting up Pi-Hole I chose OpenDNS and removed the loopback entry. making that same change "fixes" the issue (which also confirms that I wasn't accidently too agressive blocking domains, etc)
Anybody else see this before or have an idea on where to start tracking it down? As far as I can tell/see the server/container hasn't restarted and immediately started working once I changed back to OpenDNS... also odd that having cloudflare selected caused this issue.