I've have never heard of "yellow heart" as a medical condition and I can't find any references about it online either, neither in English or Portuguese. Maybe you can link me?

I think the misinterpretation makes the most sense:

Especially considering that none of the other emoji are even trying to do any kind of idiomatic interpretation of their description.

Then again, Android 4 just put random heart related shit in all the heart emoji space so maybe there really was an artist that decided to go off the rails and draw some weird hearts. Either way, they've been corrected now. Whatever experiment Google did with their emoji set, it's now over; there is some room for interpretation, but not so much that it deviates strongly from every other emoji design.

To quote the unicode committee:

While the shape of the character can vary significantly, designers should maintain the same “core” shape, based on the shapes used mostly commonly in industry practice. For example, a U+1F36F HONEY POT encodes for a pictorial representation of a pot of honey, not for some semantic like “sweet”. It would be unexpected to represent U+1F36F HONEY POT as a sugar cube, for example. Deviating too far from that core shape can cause interoperability problems: see accidentally-sending-friends-a-hairy-heart-emoji. Direction (whether a person or object faces to the right or left, up or down) should also be maintained where possible, because a change in direction can change the meaning: when sending 🐊 🔫👮 “crocodile shot by police”, people expect any recipient to see the pistol pointing in the same direction as when they composed it. Similarly, the U+1F6B6 pedestrian should face to the left 🚶, not to the right. See Section 2.10, Emoji Glyph Facing Direction.