I have had this problem as well, I recently bought my galaxy s8+ used, and it came with these annoying two black spots on the edge of the screen. From what I have reaserched, this is a factory defect.
the plus app is sometimes buggy, its best to call or use live chat support and file a RMA repair warranty order
i had the spots and it was dead pixels that only the repair facility can do.
The dead pixel issue was resolved after the local Samsung customer service replaced the glass screen (digitizer), LCD and the attached cable(s).
These are the parts they replaced.
GH82-14005A - Display LCD Touch Screen Frame + Battery Replacement GH82-14072A - Samsung Galaxy S8+ SM-G955 ReWork Kit Adhesive for LCD
Screen So the battery is also replaced as part of this (based on the part description).
I have this on a J7 Star phone...it shows even worse than on other things on the blue android software/firmware update installation screen. I agree it's ugly, but it's most ugly on that screen.