Usually it's either the digitizer touch screen being faulty or the touch screen FPC connector on the logic board needing to be replaced via micro-soldering.
Inspect the touch screen connector closely for any damage such as missing pins, plastic. Take a photo and post it here if you want to be sure.
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Edit: Right before fitting the digitizer glass in, make sure the ribbon cable is not being squished between the glass and metal.
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You can use a plastic spudger non-pointy end or equivalent to push it so that it does not squish.
Usually it's either the digitizer touch screen being faulty or the touch screen FPC connector on the logic board needing to be replaced via micro-soldering.
Inspect the touch screen connector closely for any damage such as missing pins, plastic. Take a photo and post it here if you want to be sure.