The GFYcat explains it all, basically. It does not affect other entities but the Player.
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I just tried it out and sure enough, damage. Unfortunately, due to the (extremely) scanty details that come with new Minecraft feature releases we can't be sure if this intended behaviour or a bug.
I assume it's a bug, as it significantly reduces the usefulness of slime blocks if it is the intended behaviour. To make it easier for Mods to reproduce the bug I have attached a small world with the demo machine reproduced.
Can somebody reproduce?