Okay- rather than being Northern's fault specifically, it's the fault of the stupidly fragmented system we've ended up with...
But then, the same thing happened plenty of times under BR: the Turbos having couplers specifically designed to be incompatible with Sprinters, or the plethora of different multi-working systems on the first-generation units and locos. Compare that to the United States and their mostly private system, but they still managed to have a universal standard for multi-working.
I suppose if there weren't all these peculiar quirks and oddities, this website would be rather dull!