When a creeper is launched in the air by a slime block they will start to flash as if they were to explode but the user is in creative mode

What I thought was going to happen was the creeper just to be launched in the air and come back down.
What actually happened was the creeper went in the air started to flash as if it was to explode even though the user is in creative mode.
To recreate:
1.set up a system where the sticky piston will push a slime block when activated by redstone
2.Spawn a creeper on the slime block
3.Activate the piston