ad.doubleclick.net is now whitelisted because I clicked the "Whitelist" button in the query log, It already existed in the whitelist prior to the UI button click. The only way things whitelist successfully in the UI is clicking the whitelist button in the query log.
All that being said, I ran through your test:
Adding yuxyz.com to the whitelist did make the domain resolve.
Aka I'm not able to reproduce right now so I can't help. Maybe the latest version fixed. Guess I'll be back.
But seems very similar to this previous issue:
https://discourse.pi-hole.net/t/pihole-whitelist-not-working-properly/7790/18