Its simply read voltage,Actually not. What are the other options?
https://github.com/rehius/usk/blob/6530fd9fe58980ea03d47905bad5e871b9439b7a/main.c#L64
The Voltage of dat0 must between 1.8V +- 0.2V alias 1.6V ~ 2.0V
Lets assume your flex did connected CORRECTLY to the emmc dat0 ball, then you might check the continuity of the enameled cable from it to the resistor. And check the resistance of the resistor to check whether its Open Circuit (broke).
If the flex is okay, the cable is okay, the resistor is okay, and lets assume the rp2040 is okay.
Then maybe its rare scenario, in the past its also happened,
https://gbatemp.net/threads/picofly-a-hwfly-switch-modchip.622701/page-653#post-10244019
Try to measure the voltage of the dat0 voltage in rp2040 pin.
For this scenario i have calculate based on the datasheet, should be around 1.6V +- 0,35V or 1.25V to 1.95V.
If your measurement shows on that range, maybe the firmware source code need to be change.
I attach firmware for this change if you might to try it out (v200.76)