iphone 12 mini automatic shutdown

PLEASE GUIDE:

I purchased iphone12 mini on 6th Oct'21.From 20th Oct'21,my phone was automatically shutting down.There is no fixed time,sometimes it gets shutdown in 12 hours,sometimes in 4 hours and sometimes in 24 hours.It never gets shut down when I am using it.I get shutdown,when I am not using it and when I pick it up,I find that it's shut down.I have to power it ON from the side button and then it restarts.Battery is always more than 60%,so it's not the case that phone is not sufficiently charged.Recently,when it was shut down,I did not turn it ON by pressing the power button instead I plugged the phone in power bank and phone restarted automatically.In between,I ensured that my phone and all apps has updated software,but NO use,it did not solved the problem for me.Finally,I took the phone to iphone Service Centre on 13th Nov'21 and they formatted it and updated the software,my current ios version is 15.1.1.After the update,my iphone worked well for 1 week i.e till 20th Nov'21.On evening of 20th Nov'21,again it started to show up the same problem of Automatic Shutdown.It's now again shutting down in 12 hours,24 hours, 5 hours etc.To be honest,I am fed up with this behaviour and loosing my trust.Apple support community is my last hope.Is someone else,also facing a similar issue or can someone from apple support can help me on this issue.Thank you in advance.