It seems that the Halo 3 Laso playlist achievement, A Preference for Pain, is currently bugged on Steam. It is unlocked in the Carrier page so you can check if you unlocked it there under the playlist section of the achievement stats, just FYI. This is just a confirmation post for anyone else who is confused, so yes, you are not crazy.
I too didn’t unlock the Steam achievement. It currently sits a 0.1% of players unlocked it, but I’m sure it didn’t unlock properly
I do have the nameplate ingame though