On page 79 of the English Manual are several caveats to maintain focus in Manual Focus while zooming. If the camera is set to standby lock or OFF the camera reverts to Auto Focus. You can only zoom out wide after setting focus in telephoto mode. If you do the reverse, zoom in after setting focus in wide, focus may not be maintained. Be sure you are not too close and are exceeding the minimum focus distance.

Im sure it was in MF however i may have been within the minimum focus range. The camera was about 3-4 ft from the monitor.