I made a superflat world with the Void preset. I eventually left the main stone platform and used commands to make my own. I spawned a Giant and it phased through my artificial 1-block thick platform into the void, where it died. I tried this 2 more times and it still failed.

How to reproduce:

1. Type "/summon giant ~ ~ ~" into the chat while above a 1-block thick platform made with /fill
2. Watch as the Giant falls into the void and dies almost instantly.