I have been giving your problem some thought and to be honest it is a puzzle. The only thing that I can think of and this is only because I had a problem with Skyrim recently. I got a message that an updated driverwas available ad downloaded and installed it. It completely buggered the game, it was artifactingvall over the place, I had to roll the driver back. So if you. Have te latest you might want to try an older driver.
iMac Pro review