How do we avoid approving an idea just because the prototype looks finished?
I'd review the concept before the prototype, because a finished looking version can hide how unfinished the thinking is. Ask the team to explain the idea in plain text or a simple sketch first, then judge that explanation against the brief.
The creative judgment discussion described a messy middle where ideas are still developing. A polished prototype makes that unfinished judgment harder to see. So before the shiny version, ask what the audience is meant to understand, feel or do, and what makes the approach relevant to the assignment.
The weakness could be the idea, the evidence or the execution
Compare the explanation with the brief and work out which of those it is. A rough treatment may hold a strong concept, and a beautiful one may fail to answer the question. Use the polished prototype to look at execution after the concept review, since production detail shouldn't stand in for a reason to proceed.
Keep feedback specific so the team knows what to change. Attractive work is welcome here. What matters is that the quality of the presentation doesn't decide a question the organization hasn't yet examined.
What to try this week
Before your next review of a polished AI assisted prototype, ask the team for the concept in a few plain sentences. Check those sentences against the brief first.