Hi Sam,
Apple doesn't have a way to identify which photo was marked as "not me".
However, we have an idea, if you're familiar with the photos in your Photo Library. You can search by your name to see all of the photos you've identified. Perhaps then you can remember which one is missing.
Find photos of a specific person
To find photos of a specific person, do either of the following:
• Tap Albums, tap the People album, then tap the face of a person to see all of the photos they’re in.
• Tap Search, then enter a person’s name in the search field.
Find and identify people in Photos on iPhone - Apple Support
Granted, it's not the most ideal answer, just a thought.
You can also leave feedback for Apple about the Photos app, using this link:
Product Feedback - Apple
Cheers!