Sounds (and looks) like an experience I had. The red line I saw was vertical and to the right of yours. It appeared to be the outline of the LCD as it followed the pattern of the screen.
Mine was not a big deal and I was actually talking with a rep about something else when they asked how the replacement was. I brought up what I saw, sent a photo, and they asked around and said it was not normal. It’s a bummer because you went through a replacement already, but you’ll notice that forever.
FWIW, I can see the faintest of red lines on mine (replacement) and my wife’s phone, but nowhere near what I had or you are showing. I can only notice it because I was searching (high brightness, dark environment, and it takes a while to even spot...I can’t even see it now lol).