What Ubi should do, is have optional objectives unlocked after you finish the story, or maybe just when the sequence is unlocked for replay in the DNA, maybe even for a new game after you finished the game once.
Why do it like this? So as not to piss off people who don't like seeing red Xs every time they play a mission because some arbitary bull**** is constantly hanging in the corner like an unwashed turd. Also, to give people incentive to (re)play, hell even for this game it would have worked great, the arbitary nature would have drawn players away from how they usually play and make it more of a "find the route that works" kind of thing, still dodgy but better than how it is setup now.
I know it's only really a big problem for first runners with a bit of OCD like me, but ****! Also, what's the optional objective in sequence 11 and I'll try to see if I can help. Haven't played in a week since I've lent the game out, but I did 100% sync the story.
What I'm doing is just getting thru the game, if I get an optional objective fine, if not, I'll try to catch it on the next playthrough. I take it slow just to try to improve on my gameplay. I don't see the optionals as a make/break thing. I find I'm enjoying the game more without stressing about them. As I get more familiar with everything, I'm sure I'll get more comfortable with these objectives.