Ghost touch. The most common cause is from a failing digitizer which converts touch to digital input. This is part of the screen assembly and a replacement usually solves the problem.
However, there are some cases where the chip that connects to the digitizer or its connection fails, causing intermittent issues. There were several reports of what was dubbed “touch disease” on the iPhone 6 and 6+. Apple had a replacement program for the 6+ but not the 6. That doesn’t specifically apply to newer models, but there’s a possibility of something similar happening. If that’s the case, Apple’s only fix is a complete replacement whether it’s a warranty fix or at their out of warranty price.
https://support.apple.com/iphone6plus-multitouch