This problem happens due to aftermarket digitisers grounding with the aluminium body on the inside edges. Before you install a new digitiser, put electric tape on the inside where you see the ribbon cable. Do it all around the digitiser hiding the end of the glass bit to the digitiser bit. Then apply your sealing tapes and install. I have fixed many a problems with this solution. Same goes to iPad mini 1,2,3,4. Most of these issues are grounding.