I was playing halo 3 odst in Reserva uplift because I want an ID image, a hat. To do this I have to get 14000 points, but then the game didn’t count points anymore when I was on the final. For example: I kill a brute but the game counts “0 points” no matter what.
I’m unsure of the specifics but if you spend too long on a mission you will eventually cease to accumulate points, at which stage all kills will net 0.
The only way around it is to play faster and more aggressively, taking advantage of time and skull multipliers if need be.