"iPhone 12 mini and iPhone 12 Pro Max models are not part of this program" so it seems that my iPhone 12 Pro Max is not affected to this program. Could my sound issues in games be related to bugs in the iOS?
On my iPad Air 4, I have a sound issue in a game that behaves like you are describing. It's normal for a while and then decreases to almost nothing, and then about a minute later increases to normal again. It might happen once in an hour, but it's not consistent. Sometimes it doesn't happen at all. It only happens in one game though. I contacted the developer and they are aware of the issue.
That's an option for you too unless it's happening consistently in all of the games you play on your phone. Other than that, you could try some of the reset steps (Settings > General > Reset), or perhaps take it to an Apple Store to have them look at it. It should still be under warranty.
