i apologize, my previous response was for a different iphone.
most commonly, the home button flex itself is the failure. this is what would need to be replaced, which will cause issues with the touch ID, if not syncronized with the motherboard. This is done via a horizon machine, which is only available at apple certified repair centers.
There is also a possible failure in the home Extension flex, which you said you have replaced, so unless you got a bad part, then that is ruled out.
The other probilble failure is the motherboard, Be it a connector, or integrated circuit failure. changing the FPC on the board isnt too difficult, but anything past that would require extensive knowledge of the board.
Best of luck