Not A Bug Esper Sentinel - card draw bug

[PinkMyst]

Game ID

280586651

Esper Sentinel{W}​

Artifact Creature — Human Soldier

Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays {X}, where X is this creature’s power.
The more Esper changes, the more he refuses to.
1/1

The "X" part of this card does not trigger properly. While the trigger was on the stack, I buffed the card with Mutagenic Growth, and only received one card. Next game I buffed the card with Monstruous rage, giving it the monster role which makes it a 2/2, this was done before the trigger. Still only received one card.

 

[Neo001992]

This is not a bug, the amount of cards you draw doesn't increase with Esper Sentinel's power, what increases is the amount of mana your opponent has to pay to prevent you from drawing the one card.