Update 2: Spooky Season

Official patch notes

“Jump into all-new limited-time Princess Bubblegum quests for the chance to unlock the spook-tacular Princess Bubblegum Lab coat skin.”

• All-new limited-time Princess Bubblegum quests.
• Complete every quest before they disappear to unlock the Princess Bubblegum Lab Coat skin.
• New quests drop every 2 days.

Official source

What we confirmed in-game

• The first quest, Do Not Open, only appears after you finish Buddy Up (talk to Finn in the hub).
• Princess Bubblegum stands at the Candy Kingdom gate in her lab coat and asks for 3 research boxes from the old campsite in Mount Cragdor.

Verified in-game