Just to update, I brought my phone in to the Apple Store with this issue and they swapped it out for a new one but the new one is having the exact same problem.
I decided to test other iPhone Xs models that were in store and all of them had the exact same issue from the earpiece/receiver when playing audio at high volumes (especially things like piano music).
I'm convinced this represents a hardware problem and will be returning the phone this week. I love everything else about the phone, but unfortunately, I need it to function well as a phone.