I don't think the battery is faulty.
it's just a not so good algorithm
to calculate the battery percentage.
I did the following test:
I disabled the critical battery shutdown
and used tasker to calculate the battery percentage only from the battery voltage.
I set it to shutdown when it reaches 3.2V.
The calculation is not very accurate because the voltage is fluctuating with the load current.
As the result of my test i did get pretty much the same battery life then before.
Thats the reason I don't think it's the battery
and i don't think it's a real drain ether.
The phone would have to get really hot if it converts that much energy to heat in such a short time.
So, just accept that the battery scale is not a linear one and enjoy your life