If you let the mouse hover over the definition of S, then you see that there is no reference to σ_v any more, because it has been replaced by the already known numerical value.

What you can do is

• either shift the definition of S above the definition σ_v or
  
• switch the definition of S to numeric optimization in the context menu.
  

You might wonder why the formula is stored as it is if you request numeric evaluation and all known values are stored if you request symbolic optimization. The reason is, that numeric optimization fails due to σ_b not defined and no optimization is done at all -> the formula is stored as it was input.

BTW, you might consider replacing names like σ_v by σ,v using literal subscipts. You get them by typing a dot.