You're mixing up wireframes and mockups. Wireframes should be used for exploration of ideas and generally laying down different ideas in their simplest form and

You're mixing up wireframes and mockups. Wireframes should be used for exploration of ideas and generally laying down different ideas in their simplest form and mockups (high fidelity, in this case) would then be built based on the existing components or displays new features and views. If the modal in your case doesn't support a feature, it would be caught during the wireframing, before anyone has invested too heavily into any particular design. Also, if the mockup resembled the actual UI but still differs ever so slightly, it's a sign of an inexperienced designer. Either they haven't properly made the actual functionality clear and the visuals are confusing (should it differ from the current features or not) OR the team is in need of a design system which would be used as a basis for all design visualisations and implementations.