While it is unusual I have seen this computers over the years. In fact I am working on one Apple laptop where it has a similar but slightly different graphical issue which flashes a corrupted image on the screen for a split second during Shutdown.
Connect an external display and enable mirroring. Does the issue occur on both displays or only on the built-in display? If it only appears on the built-in display, then the laptop's built-in Display Assembly is at fault. Otherwise it is likely a GPU/memory issue, although perhaps software could be interfering with the shutdown process/screen.
You only have a 90 day part/repair warranty so make sure the tech documents your visit within their service portal so that Apple knows you mentioned the problem during the 90 day part/repair warranty. If the tech won't help you, then speak with the manager/owner. You can also take the laptop to another Apple Store or Apple Authorized Service Provider to see if they may be able to assist you or address the issue.