It seems pre-selecting a mission before doing it gives an extra bit of XP on completion.
If you have “auto select mission” enabled. Shouldn’t the game switch you to the mission you are actively doing. And if completed go back to auto picking one. The trigger could be the small XP bonuses you get while performing the action (capturing, defending, guarding). To avoid switching too much it could wait for 2-3 consecutive XP of the same mission.
How it is now, I’ve seen it do the exact opposite, I select the capture CP mission cause I’m right in front of it, and halfway thru the capture, it selects another mission.