Moto G6 audio sending through wrong audio channel
So basically, I waited for a few days and it fixed itself in the middle of the night. I don't know why. Sorry, future humans! 

Given that it both spontaneously started and stopped, that it persisted through multiple restarts, and that it was present during safe mode, I suspect that this was either a hardware issue or it was embedded in the registry. 

note to editors: this answer is here simply to successfully close the question.