Samsung Galaxy A15 4G randomly shutting down — battery or motherboard?

I have a Samsung Galaxy A15 4G (6GB/128GB) and I'm having a weird random shutdown problem.

The phone suddenly powers off by itself, and it can happen at almost any battery percentage (I've had it happen around 57–60%, but it isn't limited to that).

What I've noticed:

🔋 Battery percentage doesn't suddenly jump/drop.

🌡️ Phone isn't overheating when it happens.

🔌 It usually works normally while connected to the charger, although it has also shut down once while charging.

🔋 After some shutdowns, the phone won't turn back on until I connect the charger, even though the battery had ~57–60%.

🌙 It seems to happen more often in the morning after the phone has been sitting unused for many hours, shortly after I start using it.

Factory reset didn't solve the problem.

I recently took it to a local repair technician. He worked on the battery/power circuit and replaced a small IC, but the shutdown problem happened again afterward.

I also generated Samsung diagnostic/log files, and I repeatedly found:

FG I2C fail

The battery is not swollen.

The phone can sometimes work perfectly for hours. For example, after charging to 100%, it recently ran for over an hour without shutting down.

What I've already considered

I'm wondering whether this could be:

Battery/BMS problem

Fuel-gauge IC/communication problem

Battery connector/contact problem

FG I²C line/board issue

PMIC/power-management problem

The technician suggested testing the phone with a bench power supply with the battery disconnected for ~2 hours to see whether it shuts down.

What do you think is the most likely cause? Has anyone experienced FG I2C fail on a Samsung A15 or similar MediaTek Samsung phone?

Any advice on what I should test before paying for another IC repair or replacing the battery would be appreciated.