I just upgraded to Movie Studio 18. I have 2 monitors. I have a Radeon RX 580 Series video card.
If I put the preview window into full-screen mode on either monitor, it makes both of my monitors constantly strobe/flicker, even if the video is paused. I don't have the problem if I keep the preview window docked. The clips I'm using are only 1920 x 1080. I tried reducing resolution, frame rate and deactivating effects, and that doesn't fix the problem.