Why there are artifacts on boot or shutdown?
If you have an AMD graphics card you might see some artifacts when booting or turning off macOS. It's perfectly normal and it's not a fault of the card, unless you see artifacts when macOS if showing desktop.
On boot it's the moment when our EFI firmware is handing over support to AMD macOS driver.
On shutdown a spinner might be incorrectly displayed as well in a form of some artifacts, but that's also perfectly normal and does not affect shutdown process in any way.