hi there,
bit of an issue, but when you add the cube at around 26 minutes (ish) , you then add a triangulate modifier followed by the displace modifier, at this point you then reuse the ground displacement texture, i followed this and nothing happened, after hitting buttons, adjusting the modifiers, changing stack order, leaving teeth marks on my monitor, i just gave it a new texture and BOOM! i got the same result as you did on screen, so a solution should anyone else get this problem...
i'm using v2.80.74!
thanks!