Java pipeline proto serialization does not ensure topological ordering of root transforms

PCollection<Integer> result =
        p.apply(Impulse.create())
            .apply(
                FlatMapElements.into(TypeDescriptors.integers())
                    .via(impulse -> Arrays.asList(1, 2, 3)));
    PAssert.that(result).containsInAnyOrder(1, 2, 3);