04-15-2019 08:05 AM
I asked Motorola USA about it in Twitter and they didn't have much to offer. They said something about how the phone's volume is automatically lowered after 20 hours to prevent hearing loss it damaging the speaker.
Media volume level set to maximum will normally revert to the safe volume limit after 20 hours of music playback. This is a safety precaution on mobile devices to avoid a sudden full volume blow that may increase stress level and discomfort in some people. -Mary
I told them I'm sure I've experienced the volume decreases more than once the same day.