iPhone 6S battery drain after two battery replacement unless lowpower

Hello, I am currently have a challenging and kinda mysterious misbehavior.

I bought a used iPhone 6S, the ex-owner has replaced the battery but didn't specify why, however the phone had serious battery drain, hopefully the store he replaced the battery at agreed to replace the battery again under their warranty policy. Before the second replacement the phone sometimes( about 2 times per day) shutdown and won't turn on unless connected to power. which didn't happen yet after the second replacment (4 days later).

after the second replacment:

The phone loadcycles is 0, wearlevel = 0% and capacity is 100% according to coconutBattery.

The iPhone still drain battery like crazy if the screen is on (1~2% per minute), when the phone is closed (No screen or no music) It doesn't appear to drain as much.

The battery seems to be normally discharging at a normal rate when ""Low Power Mode" is On (Like how low power mode rate should be), I don't know what the Low Power Mode is doing but it seems to stop the drain (not just saving battery like it's normal function), it goes from 1%~2% per minute, to 1%~2% per 20 or 25 minute.

There is also an even stranger problem, the battery usage information isn't showing at any time and it only says (battery information will be available after a few minutes) and it never shows.

I can feel some unusual heat under the rear camera when using the phone without the low-power mode on.

I did a full reset, and an iTunes restore without restoring any backup ( Setup as a new iPhone )

Now that I've changed the battery twice can we assume that the battery isn't the problem ? unless the service center has a faulty batch of batteries ? What does the absence of the battery information / usage indicates ? Is it a faulty U2 chip ? but the phone charges normally...

I don't have enough experience in these stuff, also I don't have enough money to fix the phone by try and error. I am trying to narrowing the scope so I can fix this. I'm also anxious about any motherboard fixes (U2 Chip) because this may damage the phone and I can't afford that now.

Please help me identify the problem, sadly the service center around here aren't helpful to identify the problem. And Apple won't touch it as it was opened many times at 3rd party service centers.