We are seeing many complaints about Bubblegum Hill being stuck once you have completed it. This is indeed a known issue that we are currently investigating. Although you are getting the rewards once dethroned, you cannot play it again since the event stays locked.
Apologies for all the inconveniences and thanks for the patience again.
As soon as we have any update on this, I will make sure you're all informed.
UPDATE 10/12/2018 - We have just managed to fix it and now you will be able to claim the throne as many times as possible. The deployment might take a bit so please stay patient.