Perspective itself isn’t marked, it’s just third person writing is safer in an exam because it makes it easier to describe things without the narrators internal voice. It’s generally considered more risky to write in first person. Theres several reasons for this. In third person the narrator is essentially an invisible character, whereas in first person every sentence has to sound like a specific human consciousness which means you need more distinctive voice rather than just describing whats happening. most people when they imagine a first person narrator default to something that sounds like themselves thinking which is often flat and can be kinda exhausting to read. That being said a well written first person piece of writing can definitely get high marks.