AOD clock’s afterimage appearing issue when screen unlock

If the AOD clock's afterimage appear and disappear suddenly when the AOD screen is unlocked, it is not an indication that the device is faulty.

This phenomenon is characteristic of AMOLED device. An AMOLED display consists of 3 color pixels, red, green, and blue, to express various colors. If you switch to a bright and colorful screen from a dark screen, you may see a momentary afterimage due to the difference in brightness.
 

Because of the instant brightness difference, the temporary afterimage that occurs when switching the screen does not affect device’s lifespan or performance.

Samsung is planning to release a new software update at earliest possible to resolve this phenomenon. You are advice to update your software to the latest version regularly.