[GAMEPLAY] Mega Gnar Decreasing Max HP

Normally, when Gnar goes from mini to mega, his maximum HP grows by an amount scaling with his level; however, I just played a game in which going Mega changed my max HP from about 4500 (mini form) to about 2800 (Mega Form). I first noticed this at about 20 minutes, and it continued to happen throughout the late game (maybe it was related to an item, or to Elixir of iron?). It made my engages pretty unreliable, and probably cost us the game (although a 5-melee vs. 5-ranged ARAM begins under ill stars).