Cannot be Normal Summoned/Set. Must be Special Summoned (from your hand) by having 6 or more monsters with different Attributes in the GYs. Gains ATK/DEF equal to the number of different Attributes in the GYs x 500. When your opponent would Special Summon a monster(s) (Quick Effect): You can banish 3 monsters from your GY; negate the Summon, and if you do, destroy that monster(s).
It's not a bug, that's the game's ruling. When a monster would be summoned with Mask Change's effect, Mask Change is still in the middle of resolving, and you cannot interrupt a card in the middle of resolving.
Due to that, you also cannot activate this card's effect against Fusion Summons that uses Polymerization or any other fusion summoning spell/trap.
If the Special Summon does not start a chain, then you can activate this card's effect, putting this card at Chain Link 1.
If the Special Summon starts a chain (such as due to the effect of a card), you cannot activate this card's effect. Effects that negate summons cannot be activated as Chain Link 2 or higher, as that would means they are interrupting a Chain in the middle of resolving.