Here is a fix for the Moto phone putting itself into Do Not Disturb mode / lowering the volume by itself:
An app called ‘Volume Lock’ by Evgeni Aizendorf (free from the Google Play store). The app allows you to lock the phone’s volume for ring, talk, media, etc at whatever level you like.
It has been installed for about 4 weeks now and I have not had any more problems. Occasionally, I see the phone trying to lower the volume, but this app keeps it from going down. A quick tap on the volume Up button usually stops this tug-of-war.
I hope this info helps others - - the problem was driving me crazy until I wondered about an app for locking volume. I probably won’t buy another Moto in the future.