I had this same problem, but I updated my Nvidia drivers to the latest version as of yesterday, and the landscape part seems to be resolved. No missing components in the editor or in game.
But now, my grass is all gone… 
Oh darn, I literally was rebuilding my nanite data for the landscape to get my grass back, as I was typing this, but then it broke again before I even finished typing this sentence…
Fun times, Epic, fun-times…
EDIT: even better, it’s broken in the game now too (always standalone-process for me):
The GOOD news, and I do mean it is that without grass, I get ~80ish in my 554 version. Here it’s 100ish, which if carried over to ‘functional’ in the sense I get my entire-landscape and grass should be a solid 25ish% increase. Performance-wise, 56 does seem to be money, but I cannot use it (currently)