iPhone randomly shutting down after battery change at Apple Authorised

I had given my Iphone 7 Plus for battery change Just a day after i started facing issue where the phone would shut down and restart at 10% battery from a level of 70% to 30% and charge back to the shut down level very fast that is faster than normal charging pattern. This problem was never there before the battery change.

I went back to the Authorised Apple Service they said this happens sometimes and if I restore the phone  this problem would go away but on doing the restore also this problem is yet there.

The phone keeps shutting down after the battery was changed at Apple authorised service centre and Apple is trying to deny responsibility of the problem that has happened at their service during change of Battery and this is not acceptable to me. There are many articles on the Internet where people tried to change battery at a cheaper rate by not going to authorised service centre and faced the same issue I did not feel apple shop would replace the battery in turn damaging the phone and deny responsibility of their mistake.

They are now asking for US$ 500 to correct the mistake they have created can anyone guide me on this issue