Battery % stuck on iphone 11 Pro

Hi everyone, I have a problem on my Iphone 11

pro started by ios 15.3.1 and carried forward so far to ios 15.4 15.4

The battery percentage often gets stuck on one number

for a long time, sometimes even hours, and then drops quickly and suddenly.

Most of the time it has heavy fluctuations in percentage (for example from 50% it

falls in a rush to 45% and so on).

I have tried every solution proposed by Apple, and

now I am "using" the phone only with the system apps and without backing

up my data, after having restored it via recovery and then reinstalled the

operating system via Itunes.

The battery life is at 92% and the phone has

been viewed by an authorized center as it is still under warranty at a

well-known chain of stores, which has not found any hardware defects. Now since

a solution is late in coming and despite various solutions (including the

latter still in progress which will be completed on Thursday through feedback with

Apple support) tested together with the very kind and competent assistance, I

also kindly ask you if you have had this kind of problems and if you have found

a solution, which does not include the usual steps which, I repeat, have been

tested several times.

Infinite thanks to those who answer.