This happens when you set the "Stair 3D and Section/Elevation" field to "Schematic" in the Model View Options Dialog.
Set it to "Full" and the Stair will be displayed properly.
Model View Options Combinations (the list on the left pane of the Model View Options Dialog) are saved with Views so it can change again if you activate/restore a View which is saved with a Model View Options Combination in which this field is set to"Schematic".
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