You have set the stair's multistory top level to None, and Revit will remove this parameter, if you want to create a multistory stair, please use Create Multistory Stair feature.

public static FailureDefinitionId MultistoryStairsHideTopLevelParameter { get; }

Public Shared ReadOnly Property MultistoryStairsHideTopLevelParameter As FailureDefinitionId
	Get

public:
static property FailureDefinitionId^ MultistoryStairsHideTopLevelParameter {
	FailureDefinitionId^ get ();
}