It sounds as though either the BLTouch isn't triggering, or there is a short circuit between the signal output and ground, or the internal pullup resistor in the Duet MCU isn't working. Suggestions:
Send M558 without parameters and check that it reports probe type 9.
Disconnect the white wire from BLTouch to the Duet (and the black wire at the same time if it's easiest, it doesn't matter). The Z probe reading in DWC should jump to 1000. If it does, that implies that the BLTouch isn't triggering, or that its output has a large leakage current, or there is a stray strand of wire shorting the white and black wires together.