He is not disappearing from the scene. The box is blocking the camera vision so it automatically moves where it can see your character.
Open up the box blueprint and select the actor in the components menu (uppermost one with the word “self” in front of it). In the details menu search for “Collision”.
In the Collision setting set the response to visibility channel to ignore. The picture I uploaded is on no collision setting, just ignore that.
BTW if you do any collision detection in visibility channel setting it to overlap instead of ignore might be a better idea.