Why my iphone14 plus was drain of battery health now 99% only bought 1 month ago?

A battery is a chemical device, and chemistry is generally pretty variable and uncertain, as well as being analog, not digital. Apple specs the battery capacity to remain above 80% for 500 full charge cycles, but that is a minimum requirement; there is no published maximum expected capacity. So sometimes batteries will perform much better than that minimum specification, and sometimes the change in maximum capacity won't be linear. 

And how you use and charge the phone can also affect the capacity of the battery. The best practice is to charge the phone overnight, every night with Optimized Charging enabled. Any other battery charging strategy is not optimal and can cause a slight acceleration of capacity loss.

Maybe you could ask your friends to sell you their phones. The rest of us all see about a 1% drop per month.

Every iPhone that I have ever owned has dropped about 1% a month. This is normal.

Take your phone to the Apple Store to have them check the phone and tell you the same thing.

Because they wear out as they age. It's normal for the battery to lose 1% a month.

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Nearly all users will experience a decrease in health of 1-2 percent per month under normal use