Why is my iPhone 15 Pro Max losing battery health rapidly despite proper charging habits?

Your phone is not losing Battery Health rapidly.

On average, Battery Health is going to decline about 1% for every 50 Charge Cycles.

So, 86 Charge Cycles divided by 50 = 1.72 or 2% rounded off.

100% - 2% = 98% Very normal.

Also keep in mind that Apple rounds down any tenths of a percent, so if the battery is really at 98.9%, this will still display as 98% in the battery settings.

Personally, I don't use Charge Limits on my phone.....because the battery will still charge to 100% occasionally anyway and somehow, users got along just fine for 15+ years without Charge Limits on their iPhones. And of course, you will have to charge the phone more often if you set Charge Limits.

I do use Optimized Battery Charging and connect the phone to the power adapter at night and leave it connected all night.....every night.

But, what I think is "best" may not be "best" for you.

Frankly, you will drive yourself crazy if you are checking the battery all the time. At 200 Charge Cycles, your battery will be at about 96%. At 500....about 90%

Your iPhone in fact is supposed to drop 1% per around 50 charge cycles. Note that the decline isn’t linear meaning you can lose a few more or less percent on a monthly basis. Also that your iPhone rounds down any tenth of a percent battery health. So your iPhone could be at 98.9% and show as 98%. I don’t see anything wrong with your battery health and suggest you just enjoy your iPhone.