Step Climb "Breaks" VNAV

[Sabretooth78 13]

I encountered an issue today where the 3rd step climb of the flight somehow caused all the VNAV speed and altitude calculations on the LEGS page to go blank.  During preflight I programmed the SimBrief steps into the LEGS and the FL330 and FL350 steps worked flawlessly.  As for the FL370 step, it was programmed to TULAG but upon reaching TULAG the FMC was still recommending FL350.  Finally, around YQD it was still recommending FL350 but with an OPT of FL367 and a MAX of FL375 so I judged it was pretty close to turning over (as it eventually did).

I dialed FL370 into the MCP and selected FLCH and it wasn't until trying to reengage VNAV after pressing altitude intervention at FL370 that I realized something was amiss as VNAV would not activate.  The ECON SPEED on the VNAV CRZ was flashing magenta and white.  I made several attempts to fixing the issue by switching between RTE 1 and RTE 2 to no avail.  Eventually I was able to regain the VNAV information by entering a lower FL on the VNAV CRZ page, at which point the VNAV information was still showing the FL370 data and recommending a climb to FL370.  At this point the CDU was in VNAV cruise descent mode, which I could not bring it out of (until eventually descending to FL330).  At this point I was just avoiding modifications in the event I ended up causing a CTD or something.

Could there be a step that I'm not aware of that I missed here?  I haven't flow the 777 a whole lot, but I never had this problem before!

Flight plan:

RKSI/33R N0488F310 SEL2A SEL Y697 LANAT Y51 SAMON Y513 KMC DCT DAIGO Y889 OATIS OTR3 PUTER/N0486F330 DCT PUGAL DCT PASRO A590 POETT/N0481F350 A590 HAMND DCT YESKA NCA13 TULAG NCA13 YQD/N0479F370 J596 YWG J89 OBK/N0479F380 J73 PXV/N0479F370 J73 BNA RMG5 KATL/08L

A couple of pictures:

In "CRZ DES", with speed intervention using the FL370 LEGS data:

http://sabretooth78.east.freepgs.com/FSX/2018-12-14-01.jpg

Upon pressing the MCP altitude intervention, the VNAV data disappears.  Re-entering FL330 restores it:

http://sabretooth78.east.freepgs.com/FSX/2018-12-14-02.jpg

Is there a suitable way to exit CRZ DES (that I was evidently not able to find) or could this be a bug?

Edited December 15, 2018 by Sabretooth78

[downscc 1,327]

It is not realistic to program flight planned steps into the FMS and then have the FMS set to automatic step climbs.  Do one or the other.  If you want to use planned steps then set the step size in the FMS to zero and manually enter the steps on the LEGS pages and be sure to append the S to the flight level (eg 370S).

In general we do not use flight plans to program steps.  The flight planning software predicts where steps may occur but FMS calculates the optimum step based on actual conditions (winds, temperature, etc all change from plan).