As mentioned above it could be an issue with the glass lens, however maybe it is an issue with the camera modules - although many parts are interchangeable between the iPhone 8 and SE, some are not. I looked at iFixit’s article and they didn’t say that the camera is interchangeable, and the iPhone SE does have a few extra camera features than the 8. In previous iPhones parts haven’t been interchangeable and if the problem persists it could be due to the camera not being cross compatible fully, (or maybe the lens housing is slightly bigger on the SE, so it doesn’t fit the 8’s housing - however I think it is more likely the camera module). [br]
Just a thought - hope this helps!