Objects disappearing half way when camera moves away

Normally my blocks look like this:

but when I move away this happens:

I don’t know why this happens.
When I look at them from the top, they are completely gone. And as you can guess, only the parts of the blocks with something behind them disappear. The strange thing is that I didn’t change anything and it worked before.

I just deleted my old flat terrain and created one with a generator.