It’s because Apple has changed QuickLook, which is used both by the Finder and Scrivener, in the last couple of iterations of MacOS with the result that Scapple boards are no longer shown in QuickLook. However, if you search the forum, I believe there is another thread on this in which it’s said that it has been fixed for the next update, whenever that becomes available.
Mark
P.S. Here you are: