If the original GRP door used two wires then the internal van wiring wouldn't suit a new ali door with a revised 3 wire lock. Maybe that's why the central locking doesn't work on that door now?
Would they bother changing the locking system from 2 wire to 3 just because they could? Maybe but sensibly they'd stick with the 2 wire because otherwise they'd have to change the switch and wiring (harness) too.
It'll be interesting when you start digging deeper