I'm not sure if this is considered a bug or if it is intentional. One of my characters reached level 25 so I brought her to deathmatch and raised her trophy helmet's def to maximum of 10. I later died and the character's level dropped to 24. I checked the helmet and it still showed a def of 10. I went back to deathmatch (still level 24), fought and claimed victory and the helmet's def dropped back down to 9.