Fix problem tagIDs cache

models/problem.ts

@@ -566,19 +566,19 @@ export default class Problem extends Model {
   }
 
   async getTags() {
+    let tagIDs;
     if (problemTagCache.has(this.id)) {
-      return problemTagCache.get(this.id);
+      tagIDs = problemTagCache.get(this.id);
-    }
+    } else {
-
       let maps = await ProblemTagMap.find({
         where: {
           problem_id: this.id
         }
       });
 
-    let tagIDs = maps.map(x => x.tag_id);
+      tagIDs = maps.map(x => x.tag_id);
-
       problemTagCache.set(this.id, tagIDs);
+    }
 
     let res = await (tagIDs as any).mapAsync(async tagID => {
       return ProblemTag.findById(tagID);