Touch IC is on the screen itself for the iPhone 6S/6S Plus.
However, the touch IC has to be properly secured to the back plate. If it moves, it can cause issues and ''may be'' ghosting. If you look at the original Apple screen assemblies, that grey IC is secured left and right to the back plate using small pieces of black tape, and to the front cam assembly using a plastic/nylon bracket. Put those back.
Otherwise to narrow it down, ghost touching is first and foremost screen, second connector. Hardly anything else.
If you had ''no touch'', or ''intermittent touch'', you would start looking at the board for display power rails.