Playing Team Slayer from Quick Play does not contribute to Stopping Power Challenge "Kill Enemy Spartans in Team Slayer Matches.
It’s because team slayer is the playlist name so you are not in that playlist so it will not count towards it.
“In team slayer matches” means in that playlist.
Why do they have team death match in quick match? It is almost like they want people to have to play game after game just to get the game mode they want. I guess that is why so many people are quitting at the start of games.
It is like they are encouraging that behavior.
I just did a Team Slayer match (from the playlist) and none of my kills counted toward it. Can confirm BTB Slayer also doesn’t count for them.