I recently upgraded my iMac to macOS Sierra 10.12 & firmware and started to see this strange screen during shutdown. It only happens during shutdown and no other time. Looks like Graphics card is fucked up? No, it's OS/Firmware issue for sure.
Yet to get a solution from Apple support. Let's see if I can get it fixed. The bad news is that there is no fix for this issue. All we have to do is to wait for next update and hope that Apple will acknowledge and fix this glitch.