iPhone Xs ringer intermittently turns itself off

Over the last month, I have missed several phone calls because my iPhone Xs ringer has intermittently turned itself off.

I have the feature enabled on my iPhone so it will only ring if I receive a call from a contact on my phone.

This issue has happened even though these calls have been coming in from persons with phone numbers contained within contacts on my iPhone and happens even though I don't have the "Do Not Disturb" feature enabled.

My iPhone has iOS version 13.3. This seems to have only started happening after this iOS was installed.

I have noticed that I have missed these calls when I have unlocked my iPhone and I see the missed calls from these contacts. I have then checked the ringer volume and have found that it has been turned all the way off. This has happened even after I have made a point to turn the ringer volume to maximum and then I have either put my iPhone down or clipped it on my belt. Also, I never turn the ringer volume off but my iPhone has been doing this itself.

What can I do so that the ringer volume will stop muting itself?