Small button assignment problem

I have setup the L button on my keyboard to control the landing lights on all of my aircraft. The only a/c that creates a problem is the PMDG 744. When inputting routes into the flight control computer, when I hit "L" it turn the lights on or off and does not input into the FCU keyboard. I had presumed when in that mode the rest of the a/c was locked out like in the 737NG. Is this a known issue and what suggestions other than using another key.