Auto step-climb not working as intended

[Busnusnu]

While doing a flight from wsss to efhk, I input the step alts as per the flight plan (FL340 to FL400) with auto step climb enabled. However, after executing, the FMS indicated that the aircraft will only reach a final level of FL360, and the rest of the step alts disappeared. In the process of trying to fix it, the aircraft did weird things like climbing to FL400 unexpectedly (I was less than 1 hour into the cruise) . I definitely did not enter the wrong way points as I cross checked multiple times with the simbrief flt plan. Am I doing something wrong or is it broken?