Played the last two weeks of season 3 (was kinda late to the party) up to lvl 77 with my burn dot crusader and had only one (1!) relic heavy flamer with 4 enchants dropping.
Overall I only had like ~5 purple ones dropping and in blue and green rarity maybe each 2-3 heavy flamers. Every other weapon for crusader dropped alot more, even in archeotech or ancient relic rarity.
Seems like a bug to me or a very poor drop rate on heavy flamers.