My iPhone dies at 30% and my battery capacity is 99%

Hi, you can check this resources:

iPhone Battery and Performance

Maximizing Battery Life and Lifespan

About the battery usage on your iPhone, iPad, and iPod touch

Download the latest iOS update, go to Settings > General > Software Update.

You may also need to restore your phone as new:

How to back up your iPhone, iPad, and iPod touch

Restore your iPhone, iPad, or iPod to factory settings

If the issue persists, bring your iPhone to an Apple Store or Apple Authorized Service Provider to get it checked. Book an appointment at getsupport.apple.com or use the Apple Support app.

Charge it more often.

Your battery health is just fine.