Hi there jonny4u,
We understand you're experiencing an issue with the volume changing unexpectedly on your iPhone 11 Pro, and we'd like to see if we can assist.
To get started, confirm your iPhone is up to date: Update your iPhone, iPad, or iPod touch
If the issue persists, remove any case on the iPhone and check again.
If you have Attention Aware Features enabled on your iPhone, this may explain the behavior: About Attention Aware features on your iPhone X or iPad Pro
We also suggest making sure Do Not Disturb is disabled and not scheduled: Use Do Not Disturb on your iPhone, iPad, and iPod touch
Additionally, check if the issue occurs without any wired or wireless accessories that have volume controls connected.
Finally, try enabling Change with Buttons in Settings > Sound & Haptics, and check for the issue again.
Thanks!