disassemble the phone (youtube).I hope you are saying the truth. my brother just bought poco m3, after reboot it does not boot. the bad thing is I told my younger brother to reboot and see what happens and the phone had 89% charge left. now I have to wait for 4-5 days for the phone to discharge itself.
just make cold or warm whole mainboard
then replug battery. boot phone with 5v charger
use it without powering off or reboot manually till i tell you exactly capacitor that is short. everyone can do this fix in home. trust me bro