TOPIC: "Must be completed" checkbox in course - problem

I have no idea what I do wrong but when a course is open to edit and I put mark on "Must be completed" it just disappears after clik SAVE button.
When I open edit once again there is no mark

I have only one CP, on course and one lesson with two questions - so it's not complicated and without it I am not able to award certificates

Is this some kind of bug? Or I do sth wrong

Hi I have the same issue. Not just the checkbox disappears, all the weightings previously inputted also all default to 100% each....Rob pls help.