Hello @methika
There is one reason that comes to mind when I think of an iPhone X or above missing true tone display. Every single iPhone’s screen is paired with the phone essentially at the birth of that iPhone when it comes to the iPhone X and up. When the iPhone’s screen gets replaced by a third party repair shop or by using any other parts other than the iPhone’s original screen, the true tone display of the device gets disabled automatically. There is no way to recover this by yourself unless you have the original screen that came with the phone and transfer the serial code using various methods.
IF you really want true tone display back and do not have the original screen, then you will, unfortunately, have to get the screen replaced and redone at Apple.