K5synth

If this a repaired display (e.g.- you replaced the backlight but it’s the same LCD module) or is this a new module here are my suggestions.

If you had to remove and solder the existing ribbon cable onto a new display module be sure you soldered it to the correct side of the module’s circuit board.

If you only replaced the backlight then i’d suggest checking the ribbon cable for damage and that it’s seated correctly on the header/connector, it could be off by one row/pin and that would also cause that problem.

If neither of those recommendations solve the problem try to post some photos.