Updata Metadata WP Rest API

I am trying to update a metadata I created from the front-end with a click event.

{
  "id": 1248,
   .
   .
   .
   "metadata": {
       "wishlist_array": [
          ""
        ],
   .
   .
   .

The idea is to add a post ID to the array, the post ID is from another post type. And it is passed on with updateWishlist() function triggered by a click event

Here is my JS:

updateWishlist: function(e, postID) {
      e.stopImmediatePropagation();

      // GET
      fetch("http://ohalocal.local/wp-json/wp/v2/oha_wishlist/1248")
        .then(function(response) {
          return response.json();
        })
        .then(function(wishlistPost) {
          var wishlist_array = wishlistPost.metadata.wishlist_array; //Getting the current metadata array
          wishlist_array.push(postID); //Add postID to array

          // POST
          fetch("http://ohalocal.local/wp-json/wp/v2/oha_wishlist/1248", {
            method: "POST",
            headers: {
              "Content-Type": "application/json",
              "X-WP-Nonce": phpVarObj.nonce
            },
            body: JSON.stringify({
              metadata: { wishlist_array: JSON.stringify(wishlist_array) }
              //Send new array to server
            })
          })
            .then(function(response) {
              console.log(response);
              return response.json();
            })

The method I created is working but it adds array inside array, with multiple levels. As follow:

["["["["["["","1234"]","1233"]","1234"]","1234"]","1234"]"]

How can I have it as a single level array?