The iPhone 6 Plus is very prone to suffering from Touch Disease. You can google it or look at my profile for a link to a blog post I wrote about it.
The quick version is that the housing for the 6 Plus tends to flex a lot, especially if you crush it while sleeping ;>). That puts stress on the logic board and tends to cause cracked solder joints on the IC's responsible for Touch.
Now in your case, you may have damaged the digitizer but that would normally be apparent. Assuming the digitizer is in good shape, you may start seeing a gray, flickering bar at the top of the screen, another symptom of Touch Disease. This requires a micro-soldering repair or you could bring your device back to Apple under the following program. It's not a free replacement but its a decent option if you don't want to have it repaired or you have a backup of your data.
Otherwise, you may want to find a micro-soldering repair shop who could diagnose your problem.