When you, a baby, or any other entity that can get 'inside' a cauldron is inside a cauldron and is pushed via piston, said entity will glitch through the bottom of the cauldron. Note that if this happens, you will be stuck inside the cauldron until you break it. Also, if you don't have the wall, you will glitch through the cauldron but also be pushed forwards, so that you end up having the cauldron face you at eye level.
How to reproduce:
1) Build device as shown in snapshot
2) Step inside cauldron
3) Activate piston with lever
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Are you saying that you can get trapped inside a cauldron? Because I don't see how that's a bug.

No, I'm saying that if you're inside a cauldron as it is pushed, you will glitch through the bottom and be trapped inside until you break it.
That's what I said, I don't think this is a bug.

Sounds like MC-89030.

Ender Dragon, are you saying an entity glitching through a cauldron is a feature, and not a bug? Sure, being trapped inside isn't a bug, but the point is that an entity FALLS THROUGH the cauldron.
Redstonehelper, looks like it. It seems this happens to all blocks less than a regular block in terms of height (ex: stairs, soul sand), except for slabs.
Your explanation was terrible, I had no idea what you were talking about. The ticket has been closed, the end.