Caveat, totally wild guess, I’m not an electronics engineer, touchscreens are AFAIK a whole science on their own, and we don’t know how the FP4’s one works anyway (there are several methods).
All we know is that there is some empirical evidence that the FP4’s touchscreen now requires the phone to be connected to some larger conductive body to reliably work – Either the user also touching the phone’s chassis, or the phone being connected to something else by its USB cable (like while charging it).
Could it be the update lowered the gain of the capacitive touchscreen so much, it now requires some kind of grounding to reliably create any detectable change on the screen’s electrostatic field?
In which case it should be simple to raise the gain back again, just enough to make this annoying side effect disappear (don’t know what this would do to the ghost touches issue though…).