fix: #9307, use _.flatten

src/categories/search.js

@@ -75,6 +75,6 @@ module.exports = function (Categories) {
 
 	async function getChildrenCids(cids, uid) {
 		const childrenCids = await Promise.all(cids.map(cid => Categories.getChildrenCids(cid)));
-		return await privileges.categories.filterCids('find', childrenCids.flat(), uid);
+		return await privileges.categories.filterCids('find', _.flatten(childrenCids), uid);
 	}
 };