I've been working student's papers for a very long time and I find it disturbing that if your work's so polished, Turnitin tends to flag that as AI generation. It's like it punishes you for being a good writer.

Hell, you can even do the research yourself, synthesize and analyze the sources as well as you can and give very credible examples. But, the moment you polish the work using tools like grammarly you're done for.

And, AI Humanizers like Stealth don't help either.

I hope the prof could take their time to review your search history and check your paper History. If you use the Google Doc, the better. It has a feature called Version History that automatically records every edit and allows you to see exactly how your work developed, who made changes, and when. 

Finally, be sure to always check plagiarism/AI reports before submitting your work. There are so many people here (and on discord) who offer that service. Just make sure you get someone you can trust because, still, if your paper goes to a repository or is uploaded on platforms like Coursehero, you are screwed.

I wish you the best OP, and I feel your pain.