Hello Rmm7987,
Welcome to Apple Support Communities. We understand that your iPhone's ringer volume changing although the option"Change with buttons" is turned off. We'd be happy to look into this with you.
To get started, let's rule out simple solutions. Have you tried restarting the device normally?
Restart your iPhone
Next, let's make sure the device is up-to-date on software. When you navigate to: Settings > General > Software Update do you see any available updates?
If so, please back up your iPhone, then update as explained in the following links:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Also, check out this helpful link:
Adjust the volume on iPhone
We hope this information is useful. Kind regards.