BUG: DataFrame.stack sorts columns

[rhshadrach]

mi = MultiIndex(
    levels=[["foo", "bar"], ["one", "two"], ["a", "b"]],
    codes=[[0, 0, 1, 1], [0, 1, 0, 1], [1, 0, 1, 0]],
    names=["first", "second", "third"],
)
s = Series(0, index=mi)
unstacked = s.unstack([1, 2])
print(unstacked)
# second one two
# third    b   a
# first         
# foo      0   0
# bar      0   0

result = unstacked.stack(0)
print(result)
# third           a    b
# first second          
# foo   one     NaN  0.0
#       two     0.0  NaN
# bar   one     NaN  0.0
#       two     0.0  NaN

The expected result should have the columns b and then a, the order in the original DataFrame.