By default, the NVR main stream is set to 720p, while your camera stream may be set to 1080p. Setting the NVR's stream to 1080P solves this problem.
To change the NVR's main stream format:
- Open the NVR's settings menu
- Navigate to Device > Device Info
- Change "Video Format" to either of the [1920x1080] options
- Reboot the NVR and the problem should be resolved.