According to the episode list June 30. apparently PB is jealous of Flame Princess.
STAY AWAY FROM TIER 15:
Romance issues Finn!
Everyone knew Marshall and Gumball didn't like Flame Prince, but who would have thought PB was the jealous type?
Not much information retrieved from episode, but that's insane: adventuretime.wikia.com/wiki/B…
The description it gives: Bubblegum may be jealous when Finn starts hanging out with Flame Princess.
Okay, it says may, but it means MORE FLAME PRINCESS!
Now all we need it to be assured Marceline is coming back...