I'm not convinced this does happen. If so it flashes up very quickly or when I'm still in combat. Not having it visible in the pause screen is a real issue as far as I'm concerned.
It always tells me what I need to do as well as what not to do at to achieve full sync at the start of each section of the mission.
Also - the mission failing thing is incredibly harsh. I've only played AC2 before and it was nowhere near this bad. You only have to be out of range for seconds and it fails you.
Good to hear you're enjoying it and 100% it, gives me a little faith.