Need help and techniques to debug DNS failure on Facebook and Instagram using Pihole with Unbound

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That means something upstream is blocking/filtering traffic with IP 185.89.218.12:

pi@ph5b:~ $ dig +norecurse @185.89.218.12 a z-p42-instagram.c10r.facebook.com.
[..]
;; AUTHORITY SECTION:
c10r.facebook.com.      3600    IN      NS      a.ns.c10r.facebook.com.
c10r.facebook.com.      3600    IN      NS      c.ns.c10r.facebook.com.
c10r.facebook.com.      3600    IN      NS      b.ns.c10r.facebook.com.
c10r.facebook.com.      3600    IN      NS      d.ns.c10r.facebook.com.

;; ADDITIONAL SECTION:
a.ns.c10r.facebook.com. 3600    IN      AAAA    2a03:2880:f0fc:b:face:b00c:0:99
a.ns.c10r.facebook.com. 3600    IN      A       129.134.30.11
c.ns.c10r.facebook.com. 3600    IN      AAAA    2a03:2880:f1fc:b:face:b00c:0:99
c.ns.c10r.facebook.com. 3600    IN      A       185.89.218.11
b.ns.c10r.facebook.com. 3600    IN      AAAA    2a03:2880:f0fd:b:face:b00c:0:99
b.ns.c10r.facebook.com. 3600    IN      A       129.134.31.11
d.ns.c10r.facebook.com. 3600    IN      AAAA    2a03:2880:f1fd:b:face:b00c:0:99
d.ns.c10r.facebook.com. 3600    IN      A       185.89.219.11

Not related to unbound or Pi-hole.
Someone else experienced similar:

EDIT: Or maybe routes upstream are wrong somehow.

EDIT2: Below did the trick seeing something wrong with the repeated attempts and no answer:

grep 'sending query\|sending to target\|response was\|reply from\|operate: query' unbound.log | less