“On volume overlap” can never be seamless. You’ll always end up teleporting before the camera crosses the threshhold.
I’ve now set it up to teleport during “post physics” tick. I then ran it frame by frame. It “teleports” in one frame, before visibly crossing threshold (as intended)… then another frame after the teleport shows up behind the portal… then the next frame is correct. It’s like it’s adding a frame in the middle for no apparent reason. 
Setting the camera to another actor will run into the exact same problem: When to transition.
I’ve tried the teleport during every tick group, with the same results.