After further investigation, it seems like it happens after the Safari tab goes inactive (15 minutes of no use, maybe?). After coming back to the inactive tab, it will look like the screenshots.
Tried it with the forum default dark blue and the default XF theme and I'm running the latest 8.0.21 PHP that it's rated for, fully patched W10 and latest Firefox. What's yours?
The devs might have a better idea of what's going on.