Why is my audio track recording my midi track?

I laid down a mellotron midi track then recorded an audio guitar track. When I play back the solo audio track the mellotron is also there. At first I thought I had made the classic mistake of leaving my monitors on while recording with headphones. Nope. I have repeated this several times now. The only input to the audio channel is input 1 from my interface. Crazy. Is this a bug?