Hey there @TankutZeytinoglu! Welcome to the community! I’ve seen this issue before and it can stem from a number of things. Often the issue is either that graphics drivers jumbling up the vertexes of the landscape when the memory changes. Sometimes this could be because VRAM is being topped off (which is not the case in your example).
What are your specs and which version of the engine is this in?
First, let’s back up your project and then we’ll try something.
After editing the landscape this way, can we try saving and closing the project and relaunching? If it looks normal afterwards, we will know our culprit a bit better.