The specification said

    Every item in I is present exactly once in S

5 is an item in I, and it is present exactly once in S.

▲ defrost an hour ago | parent [-]

and 5 is another item in I, and it's not present in S.

Yes it is, it's right there, between the 4 and the 6.

	▲	defrost 39 minutes ago \| parent [-]
		That's not the same one - track the permutation chain.