Killing a creature creates a copy of it on top of my deck

I've noticed a few times that after killing a creature during the opponent turn with a direct damage instant spell before its "enters the battefield" ability can trigger, I would draw a copy of said creature right at my next draw phase even if it's not a card included in my deck.
For example, the attached screenshot depicts such a situation: I've killed my opponent Rainbow Knights with Searing Spear right after they've been summoned, thus before their "enters the battefield" ability could trigger, and I've drawn a copy of them right at my next draw phase. My deck doesn't contain any copy of Rainbow Knights!
So, summarizing:
- Opponent summons a creature with a "enters the battlefield" ability during his postcombat main phase;
- The creature is killed with a direct damage instant spell before said ability could be triggered;
- A copy of the dead creature will be created as the top card of my deck.
I've attached a savegame even if I assume it will be of limited usefulness, as it starts on my turn instead of my opponent previous one.
Thanks.
For example, the attached screenshot depicts such a situation: I've killed my opponent Rainbow Knights with Searing Spear right after they've been summoned, thus before their "enters the battefield" ability could trigger, and I've drawn a copy of them right at my next draw phase. My deck doesn't contain any copy of Rainbow Knights!
So, summarizing:
- Opponent summons a creature with a "enters the battlefield" ability during his postcombat main phase;
- The creature is killed with a direct damage instant spell before said ability could be triggered;
- A copy of the dead creature will be created as the top card of my deck.
I've attached a savegame even if I assume it will be of limited usefulness, as it starts on my turn instead of my opponent previous one.
Thanks.