Thousand-faced shadow ninja
The problem appears when casting thousand-faced shadow ninja through its ninjutsu ability.
The card rules state:
"When Thousand-Faced Shadow enters the battlefield from your hand, if it's attacking, create a token that's a copy of another target attacking creature."
In arena, sometimes, when casting the card with the ninjutsu ability, while targeting another unblocked attacking creature that belongs to the controller, even though when the requirements are meet and the thousand-faced shadow ninja enters the battlefield, if the opponent kills, bounces or phases out the targeted creature by the ninja or the thousand-faced shadow ninja card itself, the creature is not copied, despite, meeting all the criteria for being copied.
The card clearly states that the copy effect happens if the thousand-faced shadow ninja enters the battlefield with ninjutsu. SO should be that, no matter if any of the two cards involved in the copying process are bounced, killed, exiled,... that the targeted creature is copied since the ninja effectively entered and the ninjutsu ability resolved
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[Deleted User] commented
Spells and abilities that target must have valid targets when they are put on the stack (in this case, when the 1000 FS enters the battlefield) and when the spell or ability resolves. If the target is no longer valid when the spell or ability attempts to resolve, it will fail to resolve and no part of the spell or ability will happen.