Particle settings are set to minimal
Edit: Oh wow, since the snapshots run really slow on my PC I set them down, but I never played with minimal particles. Is not a bug... sorry :/
look at the changelog:
"Axolotls currently love tropical fish, though this may change. Tropical fish or tropical fish buckets can be used to mate axolotls"
I found out that it works, and i just had the particles set to minimal, wich caused the hearts to not appear