Leonhart
Imagineer
I always assumed that Carrot was the traitor based on the Vivre Card visual dictionary data, and how her card is close to the same color as the Beasts Pirates' cards. The theory has some holes in it since it's hard to imagine Carrot knowing any of Kaido's crew since she never left Zou prior to leaving with Luffy and the others to Whole Cake Island, but she's the only character on my radar now as a possible suspect.