iPhone ringer volume keeps turning down

Since upgrading to iOS 13.1(now 13.1.2), the volume on the phone that that I normally have set on maximum level, keeps lowering on its own to about 25%. I became aware of the problem when my alerts (email, text, other alerts) were often barely audible if phone was in my pants pocket.

I can raise it back to maximum and it can lower on it's own within a few minutes even if the phone has not been in use, having any activity, or even wakened. One I time raised the volume, plugged it in to charge while in the car, picked it up less than two minutes later and the volume had dropped. In a thirty minute timeframe of checking, the volume dropped four times. The phone has now gone an hour without the volume changing so doesn't appear to be time related.

All though these events I did not use (or touch) the side volume buttons other than to raise it back to my normal levels.

Under settings/sounds & haptics, the Change with Buttons is on and does function.

[Re-Titled by Moderator]