When a diamond helmet is trimmed with netherite it can still be upgraided to netherite. When this happens, the netherite trimmed netherite helmet will still have the netherite trim.
To replicate:
open a world in 23W04a
get a diamond helmet and give it a trim using netherite
upgraide the helmet to netherite
Thank you for your report!
We're tracking this issue in MC-259356, 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