In order to maintain TrueTone, you'll need to transfer over one of the sensors on the original flex cable or find a way to transfer the serial number from the old one to the new one. I can't quite remember what sensor it is, but it's not something that can be DIY-ed. Another way you could do it, albeit it's much more expensive, you may be able to order a replacement screen assembly from apple's self service repair store. It'll come with a new proximity sensor preinstalled but you'll have to order the parts with your device's specific serial or IMEI number to program the new display.
It sucks apple designed the phone this way. I hope you can get this issue fixed.