How to eliminate nested arrays to create a "clean" 2d array

I have the below function, but it is not returning the desired result. How can I change this so it returns the desired output?

function convertArr () {

const separateNestedArrays = function (arr) {
  return arr.flatMap((element) => {
    if (Array.isArray(element)) {
      return element;
    } else {
      return [element];
    }
  });
};

const twoDimensionalArray = [
  [1, 2, 3, 4],
  [[5, 6], [9, 10]],
  [5, 6, 7]
];

const modifiedArray = separateNestedArrays(twoDimensionalArray);

console.log(modifiedArray);

// Desired Output: [[1, 2, 3, 4], [5, 6], [9, 10], [5, 6, 7]]
// Current Output: [ 1, 2, 3, 4, [ 5, 6 ], [ 9, 10 ], 5, 6, 7 ]

};

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