I had this same issue. I sent my phone to Samsung for a warranty claim. I should get it back in two days which makes the entire process take 11 days in my experience. They did acknowledge the problem and replaced the entire lcd setup. My friend also has an s8 active like mine and is experiencing the same red line in the same exact spot. It lines up with the top of our keyboard. Could be related to frequent keyboard use burning the lcd out. Who knows?!
So I am sure most of you watch Youtube on your phone or maybe listen to it as you work, drive, study, etc.... (At least I did) when I started to notice this red-ish tint that was "Burnt" into my screen's pixels. It doesn't matter if I turned off my phone or reset or any trouble shooting exercises I did via AT&T or Samsung it was still there. The only solution that worked for me was to get a new phone with warranty or whatever. Its just like they say you don't leave a TV on with no video or pixel movement. It's why screen savers on your computer are called "SCREEN SAVERS" its why where the video was playing looks fine and where the video wasn't playing has a redish tint. Next time go full screen mode and lower the brightness to keep your screen's life to last longer.
Simple solution::: Get phone replaced. (Read above for causes) No amount of trouble shooting will fix the issue.