Battery life going down fast after 18.1 update on Iphone 15 pro max (4 percent in less than a month)

The drop you will see is not linear meaning that what you see in one month may be different in the next month. It has only been expressed as an average over many months. In addition the calibration of the battery only occurs when the battery is able to charge to 100%, so if you are manually preventing that from happening, then you will not see the calibration occur. When set to charge to 80%, the phone should still charge to 100% on occasion to perform the calibration. The average over many months has been shown to be about 1% per month, so with yours at 91% you are above average after a year of use.

What Jim ID said.

Also, what is the cycle count? The minimum standard is an iPhone will lose 1% for every 25 cycles, so you can divide your cycle count by 25 to see what your capacity loss should be.