Hey FredericYue,
The reason for this could be that the camera collides with the imported objects. Most objects you import have a collision box around them, that means that you can not just walk through them but the camera will collide. If your camera is stuck in one of these objects while you hit play, it might act weird because it is stuck in the object. Could you please try and place the camera next to imported objects and make sure it is not colliding with one of them while hitting play?
Thanks!