Iphone 12 data lost due to battery problem

Dear friends,

My Iphone 12 suddenly got turned-off and none of the methods worked to turn it on again. When I visited apple store they told me that it could be battery issue and they will replace whole back panel which includes memory components as well.

So they said they cannot recover any data which left me shocked and surprised. They are suggesting to back up the data which is fine.

But if phone power system fails before we back up the data, we will lose the whole data. Anyone faced this issue before and how can I get my data back?