iPhone volume resetting by itself

I recently got an 11 pro max because I had to take my iPhone 8 to a service center. Before updating to iOS 14, I never experienced this issue in iOS 13.7 (even with my iPhone 8 that has iOS 13 installed).

Basically, I set the ringer volume around 30% to 40% so it’s not too loud nor too faint but after some time it adjusts itself to the lowest volume (0%). I raised this concern thrice to an apple support chat and they all said to:

1) Disable “change with buttons” and “attention aware”;

2) Do a full backup and restore;

3) Perform hard reset (I did this TWICE already)

But the issue’s still there! It’s honestly a headache. I keep missing important calls and notifications because of it.

I tried listing down the timestamps (and even did a screen recording as proof) whenever I check the volume but it does’t seem to have a pattern.

Does anyone have the same issue as me?