Keyboard issues remain after LCD/digitizer replacement

My wife's iPhone 5c was having many touchscreen issues - mainly with the keyboard. While holding phone upright, the N, J, U, and sometimes H keys (while is QWERTY) cannot be used. If tilted to the side, the entire keyboard would (and does) work, but this option is not available all the time (when signing into iTunes, iCloud, etc).

I replaced the LCD/digitizer (and the battery), but the problem remains. The only part of the screen that was not replaced was the backplate/screen holder.

Once I installed the I reset the phone a number of times and updates to iOS 10.2. The issue remains.

Could it be the backplate/screen holder that was causing this problem the whole time? Are there any other things I should try?

Thank you!

~Ian

Update 01/14/17

I changed the backplate/screen holder and the issue remains. Can the part that the ribbon cables plug into be replaced? What recommendations do you have?