A similar thing happens to me. I don't use Apple Music regularly. This means the last song I played is from something like 2-3 years ago. I constantly listen to podcasts and other music streaming services on my phone. Constantly. In the shower, running, in the gym, walking around town...constantly.
Yet when I enter my car and CarPlay starts for some reason maybe 50% of the time (not all of the time even) it randomly decides to play the last song I streamed in Apple Music 2-3 years ago. WTF Apple?