That log confirms your observation, so we can rule out an external component on your client interfering with DNS results (as some antivirus DNS features sometimes would).
The difference between requesting A records for IPv4 and AAAA records for IPv6 is that Pi-hole is using a cached copy for the latter. Since www.youtube.com has a rather long TTL (of about a day), this could be a caching issue.
Could you restart Pi-hole via pihole restartdns reload, rerun your nslookups and check the logs again?