Hey andre__46,
Thank you for using the Apple Support Communities! We understand your ringer volume appears to be changing on its own unexpectedly. Let's look into this more with you and see how we can help out.
Below are a few steps we'd like you to check on, and try out, to see if it can correct this behavior.
-Consider backing up your data and updating to the latest iOS 14 version available under Settings > General > Software Update: Update your iPhone, iPad, or iPod touch
-Remove any protective case from your device and test to see if that resolves the behavior.
-Do you use a wired or wireless accessory with volume control buttons with your iPhone? If so, test with that device unplugged or unpaired from your iPhone to isolate to, or away from, the volume buttons on the accessory.
-You mentioned checking volume buttons. Are you referring to Settings > Sound & Haptics > Change with Buttons? If not, try toggling this option to see if that makes a difference.
If the behavior remains, let us know if the volume indicator actually changes position in Settings > Sounds & Haptics > Ringer and Alerts, when you notice this occur.
Best regards.