fix(Util): flatten ignoring certain fields (v13) (#8936)

Fixes #8929

src/util/Util.js

@@ -41,15 +41,20 @@ class Util extends null {
       const element = obj[prop];
       const elemIsObj = isObject(element);
       const valueOf = elemIsObj && typeof element.valueOf === 'function' ? element.valueOf() : null;
+      const hasToJSON = elemIsObj && typeof element.toJSON === 'function';
 
       // If it's a Collection, make the array of keys
       if (element instanceof Collection) out[newProp] = Array.from(element.keys());
       // If the valueOf is a Collection, use its array of keys
       else if (valueOf instanceof Collection) out[newProp] = Array.from(valueOf.keys());
-      // If it's an array, flatten each element
-      else if (Array.isArray(element)) out[newProp] = element.map(e => Util.flatten(e));
+      // If it's an array, call toJSON function on each element if present, otherwise flatten each element
+      else if (Array.isArray(element)) out[newProp] = element.map(e => e.toJSON?.() ?? Util.flatten(e));
       // If it's an object with a primitive `valueOf`, use that value
       else if (typeof valueOf !== 'object') out[newProp] = valueOf;
+      // If it's an object with a toJSON function, use the return value of it
+      else if (hasToJSON) out[newProp] = element.toJSON();
+      // If element is an object, use the flattened version of it
+      else if (typeof element === 'object') out[newProp] = Util.flatten(element);
       // If it's a primitive
       else if (!elemIsObj) out[newProp] = element;
     }