using 5v-12v as input sensor

[tanoshimi]

Hmm, odd, I would have expected that to work... if you're registered on somewhere like EEVBlog you could try asking on there.  Actually before you do that can you check the voltage on DI (with the sensor connected) to make sure that it actually is being pulled down to GND?  With the various pullups and things in both devices it could be that there's not enough voltage difference across the opto LEDs to light them.  A high-impedance meter connected to the sensor output might give a different reading to the KC868 connected as an actual load.

Thankyou again for your input, I'm very grateful. 
"A high-impedance meter connected to the sensor output might give a different reading to the KC868 connected as an actual load. " -> This is my only real conclusion too.

I suppose the answer I was hoping to receive was from someone who had practical prior experience of connecting one of these sensors and could confirm "Doing it like this works in practice". And then I could be certain that it's something I'm doing wrong. And I'm just surprised that nobody has done that - rather than the theoretical "well, this _should_ work".

I'll continue to experiment. Perhaps adding some amount of pull-down resistor on the signal wire will cause it to toggle the right side of whatever threshold is required. Although I'm reluctant to add too many external components - one of the attractive features of using the Kincony boards in the first place was for everything to be on the board already. Otherwise I might have well have just spun up my own PCB....