The same thing can happen if the keyboard data line is not connected.
Or possibly if the A1200 keyboard microcontroller is not disconnected from the data line, making the external keyboard connect in parallel.
It'd be a lot tidier to just remove the keyboard micro, U13, if you know someone with a SMD rework tool.
It's then just a matter of connecting 5V, ground, KB clock and KB data to the external keyboard and you're in buisness.
Here's a schematic of the keyboard circuit:
http://amiga.serveftp.net/Schematics/A1200_schematics/JPEG/A1200-KeyboardMPU.jpg(Sorry for the long URL)
