Snapdragon damage immunity bug

This is very difficult to recreate and it seems pretty random (to me) as i do not know what causes this, but sometimes snapdragons primary weapon (fire breath) and the subsequest burning damage cannot kill players. Ive had several enemies at 1 health and had fire breath going for a solid 4 seconds and nothing happens, no damage numbers or anything. Ive seen enemies on fire at 1 health and just stand there, not taking damage. If i use another ability like the wall of fire or the secondary fireball weapon it will finish them instantly.