I remember the challenge only counting for spot assists. They also have to die while OBJ is contested, it won't count if they simply hold the OBJ. Also means they have to be on OBJ at the time, not having left it after getting spotting then dying.
There have also been changes to the spotting system in the last patch or 2, so it wouldn't surprise me something got broken.
If you have a battlebuddy, have them drop an ammo box on you while you drone and just live the drone life until you get the challenge done. Breakthrough is best for this generally though later stages of escalation can also work when 64 people are trying to cap 3 points.