Why is my iPhone 15 Pro's battery health dropping rapidly?

That's a tad excessive. Down to 80% in 1000 cycles is the minimum standard but until you get to 80% in under 1 year Apple will not respond.

Usually the curve levels out at 85% at the 2 year mark.

Charge to 100% overnight rvery night using Optimised Charging and enjoy, you won't notice the difference.

i have been using the 85 percent charge limit for around 2 week

This means that you are having to charge the battery more often than normal. Using the battery this way will result in faster battery decline.

Forget the Charge Limits and set up the battery to use Optimized Battery Charging. Connect the power supply to the iPhone at night and leave it connected all night.....every night.

Check to see which apps are gobbling up the most power by tapping on Settings > Battery and scrolling down to the Battery Usage by App section. WhatsApp.....if you are using it.....is a tremendous power hog as well as most other "social" apps

Take a look at the cellular signal strength "bars" as well at the top of the screen. If you have 2 bars or less, your phone is using up a lot of battery power by constantly searching for a stronger cell signal.

If you need to charge twice a day then I would definitely charge to 100% each time.

Not going below 20% and keeping out of the sun/heat are the two most 🌫️ important things to do.

Heavy use means heavy battery decline. That's it.