When I use Do Not Disturb on my personal iPhone, it automatically applies to my work iPhone. How do I stop this?

You must be using the same Apple ID on your personal and business iPhones. This is the reason the Focus setting is transferred across all your devices. To stop the Do Not Disturb setting from synching across all your devices, open Settings ➔ Focus ➔ Share Across Devices: Set this to OFF.

Axel F.