Translation:Isn't that difficult?
"Is that not difficult?" has to be right or at least same as "Is it not difficult?" It is about style and context.
Why not just put "is" and "n't" together? Some of these separations seem pointless or are just confusing.
It helps to test your understanding whether it is difficult or is not difficult.
"Is that not difficult" is marked wrong, but it is correct, whereas "Is not that difficult", is actually incorrect.
"Is not that difficult?" As this is correct English surely you mean that it is an incorrect translation? Would you prefer this in turn translated, "それは困難・ハードではないですか？" Is this a na-adjective (な形容詞) vs. i-adjective (い形容詞) difference?