I would think this is almost certainly a parts or installation problem. If you are seeing the same problem on multiple screens from different suppliers, then it is an installation problem. There are a million installation problems you can have on the finicky iPad mini---I wrote a blog on my iPad Rehab website about the 44 individual installation errors that I learned on my own the hard way.
In your case, I would guess you are probably not bending the digi flex toward the LCD as it is in the original. It is tempting to allow the digi flex to adhere to the adhesive on glass---which will result in the flex folding back toward the frame and ultimately forming a crease---which can cause ghosting and dead spots.
The touch ic on the screen is the cause of dead spots. The digitizer/flex connector on the logic board is touch all or nothing.
good luck!
Jessa