I occasionally get that with this site (and no other). I'm able to clear it by either closing tab and opening the site again, or closing the tab, quitting Safari, and reopening.
It's a bit easier to resolve in the latest version of Catalina, fwiw.
I don't think this has to do with Fusion (although I also have a late 2013 iMac with 3 TB Fusion), because it's isolated to this site and a single app. Of the sites I frequent, this one has, by far, the most active media content. I often get Safari warnings about this site using unusually large amounts of resources. Considering the nature of some of the symptoms (those big blocks of black and diagonal lines) I suspect it has more to do with the graphics chip of this model than it has to do with the storage system.