Uneven Battery Drain, Even After Cell Swap

Aug 13, 2016
I own my device for few years now. Around MIUI 7 update a certain issue emerged: the battery percentage has become unreliable.
The thing is, that when my phone is at 60% or less displayed energy and it turns off, when turned back on it's usually on 15% of power - the rest of which vanishes very, very quickly and the device goes black. In practice, this means that I can't reboot my device (on purpose, or worse – by accident) without a power supply.
If I do reboot when around 50%, the phone starts and displays ~15%, and connect a charger immediately – after 2 minutes battery level goes back to 50%.
Moreover, if I don't reboot and use it normally – drain pace is constant throughout.
I thought this may be due to battery degradation, so I swapped it for a new one. To my surprise, the problem persisted. Still – a reboot makes it underestimate how much energy is left.
Does anybody have this problem? Can anyone reveal the cause?

BTW: I swapped the original 3200mAh battery for a 3900mAh one, and the MIUI Security still displays 3200. Is there a way to make it reset properly?