A creeper can fall from any height (3 to 255 blocks) and not take any damage as long as they start tracking a player before hitting the ground.
To test have a creeper fall 40 blocks and stand 10 blocks away from where it will land. The creeper will take no damage and start walking towards the player. During my test the creeper was landing on rails but I believe this is irrelevant.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-19561.
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