With Preview 6, an image glitch is introduced for me, see the attachment. I have found a workaround, I have to generate optimised media and delete the render cache. The original media is a png File sequence, 1080p25. The glitches happen in the GUI and on the Decklink output.
Screen Shot 2018-07-06 at 08.14.49.png (430.87 KiB) Viewed 439 times