A small number of iPhone 11 and iPhone 11 Pro owners, as well as users of some older-model devices, are reporting a bug that causes a green tint to momentarily appear on the display after unlocking their devices.
While the issue primarily seems to affect the iPhone 11 and iPhone 11 Pro lineup, it may also show up on some iPhone X and XS models. Some reports indicate that the issue has been afflicting the displays since the iOS 13.4.1 update.
This appears to be a software bug since the issue affects both LCD and OLED screens. However, it could be a hardware bug. If the issue is a software problem Apple will likely patch the bug in an upcoming release.