HS and stacking buffs.

So as most of you may or may not know, when stacking event + card + whatever event buff (Hunter's Luck, this DB buff) makes for 8x EXP. But using HS tends to negate the effect and have some monsters give you 0 EXP (such as Newts and Nest Golems, where Newts give 8x EXP while Nest Golems give 0).

Anyone know why this happens and to which monsters in particular? Is it just transforming monsters? Or is it something else?