Array#flat doesn't work after push in one of the nested arrays

[mateuszJS]

Current behaviour: flat() is returning array with zero length
Expected behaviour: flat() should make all items flatten, so it should be [1,3,2,3], so length = 4

export function getData(): void {
  let arr: f32[][] = [[1], [2]]

  let flattenArray = arr.map<f32[]>(nestedArray => {
    nestedArray.push(3)
    trace("nestedArray.length", 1, nestedArray.length)
    return nestedArray
  }).flat()

  trace("flattenArray.length", 1, flattenArray.length)
}

link to example

Interesting facts:

• if initial input will be changed from let arr: f32[][] = [[1], [2]] to let arr: f32[][] = [[1]], then Array#flat is working correctly
• of course when push from 5 line is removed, then Array#flat is also working correctly