The same thing happened with my iPhone as well. There are many complaints that are coming up and mine is also one of them.
If you are a mobile phone repairer then you ought to know the places where you’d get a replacement piece from.
You say you’ve repaired hundreds’ of phones so I’d suggest you ask your piece suppliers to get you a replacement piece. I think that Apple doesn’t allow it’s raw materials within the reach of the general public, as it is a company that believes in a closed source mindset.
I hope you tell us the solution to the problem if you get one.
Regards.