Thanks for that. Now that I've read more about recursive DNS, I decided to see if the authoritative servers were having issues. I tried running nslookup to find the authoritative servers, and this was the result:
nslookup
> set querytype=soa
> nzb.su
Server: 192.168.99.5
Address: 192.168.99.5#53
Non-authoritative answer:
nzb.su
origin = pdns03.domaincontrol.com
mail addr = dns.jomax.net
serial = 2020102805
refresh = 28800
retry = 7200
expire = 604800
minimum = 86400
Authoritative answers can be found from: # there was no more text beyond this point
Again speaking from ignorance, it appears nslookup wasn't able to contact or determine the authoritative servers for nzb.su.
In any event, your solution worked for me, and now all of my queries besides nzb.su are using unbound.