I completed the Steppin Razor achievement requirements not too long ago today and it didn’t unlock. I double checked the recording and its in my file share now, the entire game. Also it WAS in a RANKED FFA Playlist.
Aw that sucks! I actually feel for you since I had trouble with that achievement myself. It was the only one that kept me from completing the game.
But today - after not playing Halo 3 for three years - I finally got it! The hope for good population due to the games with gold thingy and your post encouraged me to try again. After three games in Lone Wolfs, I got into a Slayer BRs game on Construct with four other players. Got the triple at main lift and the achievement didn’t pop for three long seconds in which I had to think of your post.
But it poped! So don’t give up, it’s still possible! 