Hi,
0.5m ribbon cable is a good antenna....it will receive a lot of HF noise around.
This is why nowadays every device needs to comply with EMI (not to send out heavy HF signals) and EMC ( not to be too sensitive to received HF signals. This is your case).
In your case it's just showing different characters, but what may happen with an elevator control, or an airbag control?
*****
My - simple - solution is to use a shielded ribbon cable.
My - better - solution is use differential signalling. Usually one don't want to do this with a lot if signal, therefore one tends to use a serial interface. Maybe UART combined with RS485/RS422....and a microcontroller to (re) generate the LCD signals.
For sure you can do a lot of trial and error.
* common mode choke, HF filters at the signal ends, adding a resistor at the signal end (some kind of termination to make the signal line low impedance fir high frequency signal)
Klaus