So I was trying to re-name some Sugar in anvil. I was making a potion lab, so I wanted to re-name the Sugar 'Swiftness' (without the speech marks, obviously!)
I got as far as 'Swiftn' I pressed the 'e' button, and I stopped interacting with the anvil. I tried again and the same thing happened.
If I typed Swiftness without the 'n', I could re-name it ok.
I then tried to name an item 'ne' and I stopped interacting with the anvil.
Therefore, I realized that there is a current bug stopping you from re-naming an item in an anvil that has the letters 'n' and 'e' next to each other. I hope this bug will be fixed in the next update(s).
Thanks for your report! This is however a Duplicate of MCPE-16614 – If you have not, please use the search function in the future, to see if your bug has already been submitted, as currently over 47% of tickets are being closed as duplicate.
You'll need to click back into the text input field after typing 'n'.