It is impossible to get the achievement "Treasure Hunter" with an Ocean Monument. I have tried to get it on Android and a friend tried to get it on Windows 10 and Xbox One. Neither of us have unlocked the achievement for the platforms we were trying and cheats etc have never been turned on in the world.
In the uploaded file you can see that I am on mobile, holding an Ocean Explorer Map in an Ocean Monument.
Thank you for your report!
We're tracking this issue in MCPE-45139, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki