Hi, it could be just the screen replacement itself. Often they are cheap copies of genuine products and simply do not quite cut the mustard! If all is working well and you can put up with it, carry on; if not try opening up the device again to check connectors/seals etc and possibly look to try a known working screen in your phone to help rule out any other possibilities causing the issue.
That kinda depends on whats been touched/done to the insides of the phone. I have opened/closed a couple of iPhones and taken them back to the apple store and they never knew the difference - I got replacements each time. It may also depend on the "expert" you see. I had one where they were convinced a part was different inside so decided it was void of warrenty, I then took it back another time after opening the device again, giving it a clean up and then returned to the store and got it replaced no worries!