It has two steps:
First: Randomly Generate Block at always increasing X position, that is an island foundation then.
Second: Generate more blocks to that island foundation block randomly (and then there is a possibility that it should randomly generate another block onto that generated block, which it doesn't do at all).
This is how the issue looks like, while it should make it looks more like an island and not lines stretching left, right, up and down:
The issue might be something obvious or stupid, but I am just blind to it right now.
This is the code:
Island block foundation generator:
Island block foundation top, bottom, left, right block adder: