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. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 17.0.0.0 (17.0.1090.0) 
Syntax
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