@MightyRaith my best guess is that you have a bad crimp connection in either the ground pin, the 5V pin or the control pin.
Which firmware version are you using on the Duet 3 Mini? There was a startup timing issue that affected some BLTouch devices attached to Duet Maestro boards, which was fixed in a recent firmware version (3.4 AFAIR). It might possibly affect Duet 3 Mini too although we had no reports of that.