Definitely another game trigger issue.
I just did this mission, and both the van and the gangers were there. Did the mission without issues.

Because these trigger issues are usually intermittent and not consistent (thus making them harder to track down and actually fix), best advice I can give is saving the game as a new save before activating any mission.

And also completing that mission in one sitting, not just starting it and come back later because the enemies were too hard or something.