We are currently testing a fix for this issue, once confirmed it will be released ASAP.
The plan is that once this is patched through, it will make any dead ends that were intended to be a Treasure Guardian appear as one. If this dead end has already been cleared, then you won’t need to complete the battle again, you can just collect the treasure.
Once we have any more information or should the detail of the fix change, we will post again.