The view that you have placed on the layout has the MVO, Graphic Overrides, Layer Combination, Renovation Filter and other options saved with it in the view settings.
These settings may be different to the plan view that you have open.
Select the drawing on the layout, right mouse click and choose 'Open Source View'.
This will open the plan with the same settings as in the view settings.
If the stair is not there then you need to adjust the settings saved with the vie.
I would be guessing layer combination or renovation filter.
If the source opens and the stair is showing then I will have to think about it some more as I can't imagine what it would be.
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