I bought my Xperia Z3+ back in 2015 and I've been very happy with it. Lately though, the phone started turning off or restarting when the battery reaches around 60% or I use a "heavy" app like Google Maps or such. I tried restoring the phone to factory settings, calibrating the battery, and using the companion on PC but it still didn't help. I get a "Battery health is good, there is no need to replace it" prompt when I check the battery health using the *#*#7378423#*#* test menu. I removed the SIM card and SD card but the problem wasn't resolved. The phone does not turn off and works totally fine when plugged into the charger or a power bank.
So, I finally sent my phone to an official Sony repair and they said that the motherboard needs a replacement for a ridiculous fee since the warranty period is over. Now my question is: How would the phone work if the motherboard is faulty? I feel like they're trying to get money out of me. Why would the motherboard fail all of a sudden? Is there a way to check if the motherboard is really the problem? I didn't drop my phone and there is no water contact so it can't be that.
Any suggestions or advice would be great, thank you in advance!