Multiple reports were received regarding an issue where progression became impossible after defeating the 15th monster and receiving the reward. The cause has been identified, and the issue has been resolved in the current version.
Update: This issue has been resolved as of the 2:00 AM update, as the fix matches the implemented changes.
Cause: If the reward items granted every five stages were out of stock, nothing could be distributed, resulting in the progression issue. This has now been fixed so that when the designated reward items are out of stock, players will receive gold bars instead.