Disconnect the screen, check the whole screen for any faults.
Disconnect the battery, wait for a while the reconnect the battery and then reconnect the screen
If that doesn't work then I've got no idea.
I've only ever done like 10 iPhone 6 LCD repairs but they've always been so smooth and no problems whatsoever
It is worth checking the screws for LSD.
The bottom 3 screws should be smallest
top right should be big, top left should be bigger then the bottom 3