Is Quillbot slipping? Help me find the best AI humanizer after a Turnitin flag

Posting this here because the Quillbot subreddit looks pretty dead and I’m out of places to ask.

I’ve been using Quillbot’s AI humanizer on and off for a while. Mostly for small rewrites whenever a paragraph felt too Ai-ish. I always assumed it was one of the safer tools out there, maybe even the best free AI humanizer if you needed a quick fix. And it worked for a while actually. My past papers never raised any questions.

But this week something changed. I ran my draft through Quillbot’s humanizer the same way I usually do, checked it for weird phrasing, and felt confident it looked natural. How naive of me.

As you already might have guessed, my professor ran the paper through Turnitin and the AI score showed it was AI-generated. So, of course, I had to rewrite the paper fully on my own, and now the professor is suspicious of everything I submit.

Maybe I just got unlucky, idk. Maybe Turnitin’s detection has improved or Quillbot’s humanizer has changed.

Anyway, what is the best AI humanizer today that works well consistently in your opinion?

Has anyone here had a similar experience with Quillbot? And if you use a humanizer by Studyagent, how reliable has it been for you so far?

Disclaimer: I’m not looking for promotions or debates, I need to understand what went wrong and avoid repeating the same mistake because I can’t afford another warning from my professor.