That is because Pandora is selected as the default Google Assistant music application.So when I use voice activation to access a song, with AA active...Google always wants me to access Pandora, which I am not signed into. So I have to say, "Play Hot August Night on Samsung Music" and it works.
Is this the way it is supposed to work?
To change it on your phone, first disconnect from AA and then press and hold your home button, or say "OK Google" - depending on how you have your phone configured - or however you launch google assistant.
Once the assistant is running, you may need to swipe up to get the full-screen profile view.
From there, click your picture (or whatever you have in the upper right corner) to get to the settings for Google Assistant.
Scroll down until you find "Music".
Choose "No default provider"
Back out to save the change.
Now, when Samsung Music is full-screen in your vehicle running on Android Auto, you should be able to say something akin to "Play Hot August Night" (after hitting the assist button, of course) and it should tell you that it is asking Samsung Music to play that track.
At least that's how it works in >my< Corsair.