When you add "Receive a grade" competion condition to a forum you can't remove it.  To reproduce:

1. Create a new forum
2. Set Whole forum grading to Point
3. Add Completion conditions.  Firstly add Start discussions or post replies = 1 and then also that student must Receive a grade "Whole forum" and choose Any grade. Save.
4. Return to settings, Uncheck "Receive a grade". Save.
5. Return to settings, Receive a grade is still checked off.

This is possibly related to my other reported bug MDL-82026?  I only wonder that because it seems in both cases we are "unchecking" an option and the payload being sent to the server doesn't include something which says 'option = false'.