First insure that the display connector is good.If you insert iPhone 7 LCD on 8 plus the maxium chances is that the connector might be loose that causes may that issues.
If the connector is looking good in a condition check the Gnd resistance on LCD connector or might be the problem of Chestnut IC and light IC too.I also faced that same problem like you and i solved it by this ways replacing chestnut,and two light IC.