It happened to me (I noticed it happen on the map Trainyard) that when doing the ‘secondary objectives’ missions I didn’t have to fully repair the side generator.
I just did a little bit of repair but was interrupted by an opponent so I had to let it go and fight. I didn’t have the chance to go back and finish it, a team mate did it, but nevertheless it counted towards the mission.
It looks like a bug because I only repaired a very small percentage of it.
I remember talking about this a while ago, can’t recall exactly what was said though.
Either it used to give the mission to whomever completed the most, or whomever finished it, but people complained that other were stealing their missions, so they changed it.
Or it was meant to work as I just described previously, but they didn’t have time to implement all the logic for it yet.
I’m not sure, but either way, sounds like it needs revisiting again.
If it was the former rather than the latter perhaps a compromise of anyone contributing more than X percentage gets mission credit, something in the realm of 30-50% perhaps?