Missing substitution in the simplifier?

When creating a binding (in the function completeBind), we simplify the unfolding in the binder before making a float (in the case we don't inline anyway). However, it seems that the newly obtained final_bndr is not substituted:

      ; let final_bndr = addLetBndrInfo new_bndr new_arity is_bot new_unfolding
      ; if postInlineUnconditionally env top_lvl final_bndr occ_info final_rhs
        then […] -- abbreviated
        else -- Keep the binding
             -- pprTrace "Binding" (ppr final_bndr <+> ppr new_unfolding) $
             return (mkFloatBind env (NonRec final_bndr final_rhs)) }

(See in full context here)

This is awfully suspicious: the new_arity and the new_unfolding will, I understand, not be available at the binder's use sites (which is where they are to be used).

I don't know how to make evidence of the suspected bad behaviour. And maybe I'm just missing something, and everything is actually fine here.