This past Friday, I made an appointment at the closest Apple store near me Pittsburgh (about 60 miles). I stated my problem, they acted surprised (who knows, maybe i really was the first one they seen with this issue? Maybe, I dunno. Or, more than likely, Apple does not want their employees to acknowledge a somewhat known issue to the customer). They ran some diagnostic tests from an iPad. The tests came back "phone working 100%" even with the earpiece speaker not functioning when playing a YouTube video before the diag.
In the end, the very helpful "Genius" (My god, that title. Nothing derogatory implied towards the person behind that title but....really Apple? Genius? How about something like Apple "iTech"? At least that title sounds a bit less arrogant for your average Apple store employee.) swapped my phone out for a new one. She was very easy to talk with and even managed to move my screen protector from old to new iPhone. That Apple store gets an A+ for overall satisfaction, from me.
So far so good, I haven't had the issue come back. I've been trying like **** to reproduce the symptoms. Before, I had to leave Bluetooth turned off or I certainly would lose the earpiece speaker within a 24 hr period.
I dunno people, maybe the combination of a new (warranty swap) phone and iOS 12.1 has remedied the situation, who knows. Time will be the judge. If it comes back, I will report back but for now I'm happy.