Void warranty because unknown reason

First i serviced my iPhone 12 pro max at authorised apple service centre, the problem was the microphone didnt work.

It was covered by warranty, so they replaced my iPhone back system and change my serial number, but they still used the old camera part.

Problem on microphone solved, got my iPhone back.

Then after using for 2 weeks, when i wanna use flashlight it didnt work. So i returned my iPhone to the same authorised apple service centre, after almost 3 weeks they told me that my iPhone warranty is void because there is an unofficial modification on my iPhone.

I dont know why, because I only service my iPhone just that one time and I bring it to the authorised service centre.

Could someone help me with my problem?