black display defect that spreads slowly

Unfortunately, I have had a black image error (with a light blue border) for quite some time, which slowly spreads from the top across the display. Depending on the app on the Watch, it is visible and annoying. It is best seen when using the lamp function. Unfortunately, I do not know where the error comes from, as there was no external force (no fall, etc.).

Apple Watch Series 4

Version 7.3.3