Since FM version 5.6.9 (22954), display errors have frequently occurred in the preview, especially or better said almost exclusively in the use of snippets. Image content is overlaid by black areas, images are wrongly put together like #6 in my example collection, where you can see the spire below the church.
I’ve had similar issues before, but usually if I stopped for a few seconds and started the preview again everything worked for me. My guess is, that there ist too much image data that needs to be loaded; especially if you use snippets with multiple layers. I would try to reduce the pixel-size and file-size of the images for the snippets.
may be a solution. but worked without problems in same picturesshow begun in previous version MacOS 10.14 and FM version 5.6.8