I had this happen too - on a field that had worked fine for over a year, then suddenly added fees. The fees were the same as those of another field, which I found interesting.
Here's how I solved it:
1. check fee field on which will make the fees editable.
2. delete the fees and save
3. go back in and uncheck the fee field, and save again.
That should resolve the problem.
I have been trying all of these work arounds posted but none are working. I have extra fee only showing showing up on non fee items only on the Group registrations. Anyone have any ideas on how to fix this.
Anyone having this issue... download and reinstall the latest build. Issue should be solved where sometimes a text input field would save while storing fees from another field.