It only seems to happen in the real app I am working on and not within any of the test apps I make.
Its lasted a few Xcode versions. Don't know if this is widespread to the point of having a name?
Is there any way to get around this Xcode previews bug?
I've never had anything like this happen to me.
Do any of the buttons below the preview make any difference? Zoom in/out, appearance customisations, etc.?
Have you deleted the Xocde app completely and reinstalled?