While the IOBoard signal is 0V/12V, lightbulbs would die quickly if they were to be turned on and off this much. For that reason the power distribution board generates a keepwarm voltage and the signal becomes 11.86V/12V.
For that reason, replacing the lightbulbs with 12V LEDs make the buttons always lit (goes from mild lit (11.86V) to lit (12V)).
The keepwarm voltage is generated by 9 resistors on the power distribution board labeled R1 to R9 (one for each button, the 12V goes through the resistor (dropping to 11.86V) then back to ground so there's a constant 11.86V path from lamp to ground. When the IOboard sends the led signal (ie. shorting led gnd pin with he 12v pin), a new least resistance path is created and the lamp gets 12V.
You can either desolder these resistances from the power distribution board, or bypass by wiring CN2 connector from BemaniPC side directly to the plugs which goes into the CN8 connector on the power distribution board (see above picture, unplug the circled 11 connector from the power distribution board, then plug dupont cables directly inside that connector which runs straight to the control panel lamps).