Solution: YouTube Music not recognized as default
I have some HomePod minis, and the OG HomePod. I could ask Siri to play something, and it would play from YT just fine since I had that as the default music. But for other family members, it would say this is not in your Apple Music library. Siri recognized the voice, and YT was set as default in their Home app too, and are a member of the Home I created. Well turns out the OG HomePod is the culprit. Once I unplugged that, it works fine now. This also seems to have fixed another problem. Whenever I would ask Siri to play something, it would always repeat the first song. So there must be some glitch with the original HomePod, and the new feature to use a different service besides Apple Music. Happy Listening!
HomePod, 17