just joined a game, round 1 was 100 grunts, grunts ceased spawning at 72/100
waited for game to end, game ended and it shows me down a boost. Will I get that back later or does that still count as a failed game?
If you used the boost and you failed to complete the objective you loose the boost. So yes, you lost the boost.
The game would have credited you the XP from the boost even though it was a short match. There are no situations where you can “lose a boost.” Your base XP would not be as high as it would had the match been longer, however.