Given the things we know the traitor revealed to the enemy and the timeline of events Kanjuro is the one that makes most sense, he was probably a traitor even before the time jump.
So let me go over a few points:
1-Jack learned 2 things from the traitor: how to get to Zou and the fact that Raizo was there. Back when this was revealed most people suspected that a mink was a traitor, but Kanjuro could have provided them with this information as well. The samurai had a way of getting to Zou (probably Inuarashi or Nekomamushi’s Vivre Card), and Kanjuro knew that Raizo ended up there. Add to this the fact that his ability would enable him to send messages (and the Vivre Card) using his painted creatures.
2-After he was “captured” by Doflamingo’s men. He spent the next few weeks just sitting in a cell. He wasn’t sent back to Kaido or anything? Kaido sent an army led by one of his top men to get Raizo but he just let Joker hold on to Kanjuro?
3-The first time the traitor revealed the alliance’s plans in Wano it could have been anyone, but the second time among the information revealed was the fact that Hiyori is alive and in Ringo, not many people are privileged to that information – Zoro, Kawamatsu, Kin’emon, Raizo, Kikunojo, Inuarashi, Momonosuke, Tenguyama, Kanjuro and some other Straw Hats (some other characters may have learned it off screen but that is not relevant). Through the process of elimination and considering the points 1 and 2, Kanjuro stands out as the most suspicious.
4-This is more of a meta reason, but Kanjuro is the most underdeveloped of the Scabbards even though he was the second to be introduced. The traitor plotline would work well to expand his character and add extra conflict to the Scabbards.
*Theory by Wurdlak17