Image tilt is not unusual with ball heads and due to its special design, it is inherent to Uniqball, too. The optical axis slightly rotates up or down by about 0.2 degrees while the red knob (see No.7. in the manual) is being secured. The longer your telephoto lens is, with higher magnification, the greater is the resulted image tilt in the viewfinder. This phenomenon may mostly be avoided by precisely balancing your equipment on the head, so that the red knob requires only a gentle touch or even no adjustment to achieve a proper level of friction that allows the lens (actually the inner ball) to move easily while also providing it with appropriate stability to stay in place when released (or left alone).
Remark: Please bear in mind that the level is not changing, only the horizon goes slightly up or down.