No, that's not it. I am aware of the "Close" vs "End Mission" difference, but in this case not only the progress is lost but in addition it drops me back to previous mission.
For example, after I succeed in mission 1, I advance to mission 2. Then if I die in mission 2, my score will be 0 like in the attached screenshot. 0 means go back to previous stage, so I have to re-fly mission 1.
From the User's Manual (Campaign Builder section):
So, in sequential campaign, a mission should have a score of 50 on failure and more than 50 on success. Anything less than 50 will result in going backwards, so should never be used (in a sequential campaign).