We use Figma at work, and while it’s a nice tool I think it’s easy to fall (or maybe we just fell) into a kind of uncanny valley. The designs we get are close to realistic mockups of our actual UI, but are often not 100% there, and it’s the little deviations that can be quite distracting. Engineer: So are we changing the nav icons as part of this story, too? Designer: Oh, no, ignore that. I didn’t have our real icon…
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.