Why does my TV have black line on the screen?

I have Samsung 49KS8000. I bought it in Nov 2016.

I watched the movie yesterday, and I noticed the black line on the screen. I inspected closely and found few lines. These lines are barely noticeable if I sit far. The problem is black line.

I called to Samsung Support. The rep suggested me to do picture test. I confirmed that I still see black line and few faint lines during the test. We tried to unplug it off and waited for 30 seconds then plugged it in. The lines still are there. We tried to use factory reset and it is still there. The rep assumed it might be hardware issue and suggested me to have request service.

Actually, I notice the LED light strip on the back of TV comes on at once after I turn TV off. It will be automatically off within 10 seconds.

It happened often within one week. Before that problem started, I saw the white flash on the screen often. It stopped completely after I turned TV off then turned it on. The black line was not there, though.

How can I fix it?