I was using a Level Sequence for making a cut-scene and noticed this behavior.
Looking directly up or down while piloting a camera in a level sequence appears to gimbal lock the camera
What you are seeing above is an issue that occurs in the general editor as well, and has been bugged. If you would like to keep an eye on the issue, you can do so here. If you have any other questions regarding the topic above, feel free to reopen this and I will get back to you.