sleep/wake via power button broken after botched repair

Okay heroes, I am stumped.

My shop had an iPad Mini 3 (A1600 - Wifi/Cellular) come in after a botched self-repair.

I have done a visual inspection and talked extensively with the customer: They were trying to repair a cracked digi and cut through the home button flex. Thankfully at that point they stopped without opening the iPad the rest of the way, so as far as I can tell the cracked digi and fubar'd fingerprint scanner are the extent of the damage.

Here's the issue: after replacing the digitizer and installing an aftermarket fingerprint scanner (*sigh* goodbye TouchID) everything works fine except for the sleep/wake via the power button. I can hard reset the iPad with power/home and I can shut down the iPad with a long-press, but tapping the power button does nothing.

I encountered this exact issue with the iPad Mini 4 after LCD/Digi repair and that was because I'd missed soldering on the magnetic induction flex. The iPad will not sleep/wake with a magnetic cover, so I suspect this is something similar, but my Mini 3 experience is lacking and I'm not sure what I'm missing.

Anyone with more Mini 3 expertise than I who knows this issue, I'd be eternally indebted if you have tips on where to start troubleshooting.

(WHAT I'VE ALREADY TRIED: several different home buttons, several different digis, replaced power/volume flex, DFU restore, searched settings for sleep/wake options--nonexistent)