That's happened to me a couple of times on Firebase Vancouver if I get the drone escort objective. Midway through, it'd just suddenly vanish along with the timer, resulting in us being unable to complete the objective which is especially annoying if it's during Wave 10 as the only option is to bleed out and cut our losses.
Happened to me once, I was invisible. I played the whole match lol. I could NOT damage enemies at all but I COULD revive my teammates. So I was basically medic for the match hehe. --- http://img202.imageshack.us/img202/6932/barcelonacrest.png