Hey would you be kind enough to check the iphone 8 plus model you returned. Which model no. was it for e.g. A 1897, 1863 etc. Want to see if there is a faulty batch on a particular series.
To clarify, even if you could pinpoint a particular issue with your iPhone 8 based on another model with your theory, that doesn't mean it's a widespread issue with that specific batch. Just Because what you experience with your iPhone 8 for that specific model, doesn't mean it's going to happen to the next person who has the same model iPhone As you.