I'm a little freaked out right now because I tried recreating this and the cube is still there, nothing different about it. When I look to my right, there's an even higher above object that gets lower when I move forward into it. But the other object is gone, nothing there when I move to where it sinks to as far as it can go without disappearing. The sky turns black when I look into it, don't know if I already told you that, but this time the other object only appears in the black, it's gone when I look outside of the black area.
See for yourself:
https://imgur.com/a/Rl6Gm
