Letters on keyboard are not showing

Hi,

I changed a screen on a iphone 6s plus, done 100’s of screen changes. the screen i took off was a aftermarket one. i replaced with a genuine apple screen pulled from icloud locked device. For some reason now though the letters wont show on the keyboard. the keyboard comes up but the letters are all blank. by doing a reset it overcomes the problem, but the keyboard will go back to not displaying letters again after a while of use. Touch all works fine on screen, no problem of apps opening by themselves etc at a loss myself. Please help

Thanks