Gambit’s Flak Jacket reduces the damage from Bleed effects by 150% while active. However, here it seems he is not, for this bleed passive on this node:

Here is a clip of him clearly taking the bleed damage:

I could be missing something, but I don’t believe the nodes or sabre’s kit affects bleed potency enough for him to take that much damage from it, it’s supposed to be -150% potency.
I’m guessing it may be that it’s a new node, and so certain champion resistances have not updated yet to interact properly (as it is still a beta). Either that or Gambit’s bleed resistance is only for Bleed Debuffs, not all bleed effects, which I haven’t tested but could be the reason (although I personally think it’s the first reason).