How to tell if earpiece flex has been replaced?

Okay, I'm starting a new answer because things seem to be getting slightly confused here and I'd like to clear it up.

What we've got going here are actually two separate issues.

1. True Tone is not working, despite being programmed into the replacement screen.
2. Face ID is non-functional.

First, let me make this perfectly clear: the two problems are not related!

True tone data is programmed by plugging the OLED connector into the programmer, reading the data, then programming that data into the new screen. So the programming is solely related to the LCD, not the digitizer and not the earpiece speaker assembly.

Presumably you've tried copying the screen data from the old screen and programming it into the new screen, which isn't working. Have you attempted to read the data directly from the phone using 3uTools or JCID software? That would be my next step. Also, once you've programmed the screen data, immediately read it back out of the phone and verify that what you're reading is what you wrote. It's always possible the screen is acting like it got programmed, but didn't actually accept the data.

Since I'll be doing this soon with my grandson's phone, I'm invested in helping you figure this out.

Okay, on to the second issue, Face ID. Here are some pictures of a genuine Apple earpiece speaker assembly.

You'll note there's an Apple logo on the end of the connector, although its presence doesn't necessarily prove anything since Chinese imitators frequently copy logos and part number information on their reverse-engineered parts, but if it's absent that would be a strong indicator that it is a replacement flex.

As far as the data programmed into the earpiece speaker parts, that's entirely different from that used in the True Tone information. For True Tone, it just has to have a serial number that matches the data in the phone, so a simple copy operation fixes that problem. With the Face ID, that data is generated by a proprietary encryption algorithm and presumably includes unique data from the earpiece assembly and the phone itself. As such, it's impossible to recreate without access to that algorithm, which Apple doesn't even share with its authorized service centers; they can use it remotely to do Face ID repairs, but they never actually get a copy of the program.