I hope this is the correct place. I've been having problems where debian sends me to debian.map.fastlydns.net and I get an ip. Depending on where one is located one would get a different one. The problem is the one I'm issued to get my updates from times out.
I used the link below to see that depending where one is, a different ip is returned.
I found one in the US that I could ping so I used my host file to force a substitution. That let me do an update but I have 10 more computers and I'd rather just put the substitution in pi-hole so there is only one place to deal with.
You could try create a debian.map.fastlydns.net. DNS record via the Pi-hole webGUI and point it to that US based IP you found.
Of course if Debian or Fastlydns decides to change any, it breaks again.
I did the pi-hole dns trick to get by. My ISP has managed to get someone at fastly that is taking an interest in it. I'll update the thread if it gets fixed correctly.