Need help with broken Landscape

Hi everyone. I was working on my map so nicely, placing grass, trees and stuff, painting terrain and so on. Suddently I turned the camera and this is what I saw. A piece of my Landscape (built with world composition) is broken. No collision and foliage is all on the water plane level and sometimes the landscape squares are flickering like visible/invisible. Well since I’m working on this project for a long time and I have a deadline to complete it, could someone please tell me what the hell is happening here? Is there something wrong I could have done to cause this issue? is there any fix?
Any answer will be really really appreciated.