At times my ringer volume turns lower on its own. How do I prevent this?

My ringer volume keeps turning down even with face recognition turned off as your fix has recommended. What other things can I do to fix?

If Settings > Accessibility > Face ID & Attention > Attention Aware Features is on, the ring volume will automatically decrease as soon as the phone recognizes that it has your attention.

Also see: Turn Attention Aware features on or off on your iPhone or iPad Pro - Apple Support