Hey there Vhaele,
Thanks for reaching out to Apple Support Communities. I understand you’re having some issues with the ringer volume on your iPhone changing on its own. This started after iOS 13 was installed. I’ll be happy to help with this.
To verify, are you adjusting any volume on your iPhone before this occurs? Sounds for music or when watching a video? This may isolate the cause of this if so. Also, if you have any Bluetooth devices, disconnect those from your iPhone and see if the issue continues.
If that doesn’t help, go to Settings > Face ID & Passcode on your iPhone and see if Attention Aware Features are on. If so, turn them off and then test again.
Try restarting your iPhone as well:
Restart your iPhone, iPad, or iPod touch
Let me know if this helps.