Good afternoon Snebold,
Welcome to the Apple Support Communities!
I understand you are having an issue where volume for alerts and notifications keeps changing. You have set the volume using the slider in the Settings app. (Settings > Sounds & Haptics). Have you turned off the "Change with Buttons" option? Restart your iPhone after making changes like this.
If the issue persists, you may need to back up your iPhone and restore it to factory settings.
Set up the iPhone as new, enable any iCloud features you were using before, and test for a few days with this new setup.
(Contacts, calendars, notes, etc. that are enabled for iCloud would not be in a backup because the data is already in iCloud. This data will populate to your iPhone when you enable it in the Settings app again).
How to back up your iPhone, iPad, and iPod touch
Restore your iPhone, iPad, or iPod to factory settings
Set up your iPhone, iPad, or iPod touch
Change your iCloud feature settings
If this resolves your issue, you may wish to keep the new setup. If you choose to restore from your backup and the issue starts again, then you know there was something in the backup that was causing this.
Take care.
