This black line will appear in the middle of the screen even in game mode, this will only happen if you’re using (exponential height fog) the way to get rid of it is to change the (fog height fall off) from the details tab, you can get there by clicking on the actor from the world outliner. 0.05 works for me, results may vary so change the (fog height fall off) until the line is no longer visible. cheers!
I just wanted to put this out there in case anybody encounters the same problem.
Thank you, I was having this problem