It's an issue with VA panels starting cold. You probably notice that it's more common when the temperature in the room is cold.
If your monitor has a startup graphic when powered on, or its settings menu appears at the bottom of your screen/overlap with the affected area, you'll know the issue is with the panel and not your gpu/cables if the artifacts appear over top of the menu.
My MSI Optix G24C has the same issues (uses a VA panel manufactured by Samsung), and can take a couple min to warm up if the room is cold and hasn't been used in a while. The issue typically goes away in the summer/resolves itself faster.