Optional radio button value saves as default value, if N/A selected

I have an issue with optional radio buttons saving as the default value on first save, when N/A is chosen, and the default value is something else. Steps to reproduce:

1. Create a new content type
2. Add a new field: List (text), Check boxes/radio buttons
3. Make it optional
4. Put "1|1" and "2|2" in the Allowed values list
5. Set the Default value to "2".
6. Create new content, and choose "N/A" for the field's value.
7. The field will save the value as "2".
8. Edit the content, and change the "2" back to "N/A". The field should now save properly.

From my understanding of Default Value, it should simply default to the default value when presented, but not change to it, when saved with a different value. Also, if the field is set to required, and "1" is chosen on first save, it correctly registers as "1".