Strange Vnav Altitude In Sp1

I have encountered (most likely) a bug related to altitude today while on ILS final approach:

1. The aircraft was near the IAF.
2. The A/P modes are SPD, LOC and FPA, with G/S mode armed.
3. The FPA is set to descend to 3000, where I wish to capture the G/S for auto land. Unfortunately the aircraft was still slightly above when it pass 3000.
4. Then I set the target altitude to 0 on the MCP and instinctively press the altitude nob. The aircraft was at around 2940 at this time. Note that VNAV is neither engaged nor armed.
5. Now the bug happens:

• The landing altitude warning appears. (I noticed this happens the instant I press the nob)
• The descend profile in the navigation display shows that the ideal descend path is below. I am 1000000 feet above it! (Yes, one million feet!)

The A/P was disengaged and I hand flied the aircraft to capture the G/S, then re-engaged for auto land. It landed successfully so I don't know what might happen if I to GOTA then select VNAV.

I have a screenshot of the approach, but I don't know how to upload it here (I suspect I don't have the privilege to do so). It shows the aircraft 957142 feet above the ideal vertical profile, while at 230 feet on the ILSZ34L approach of RJTT.

I am using SP1 of the 777 by the way.