I have the same problem with my Motorola Moto Z3: Low volume after the last android update and no debris in the speakers. Was this issue ever resolved? Based on Verizon's response to others doesn't look like it was resolved.
Having a volume level that you can rely on is essential. I would like to assist. Do you experience an issue when on speakerphone too? Or is it just the ear piece? When did this issue first occur?