Attack cooldown on tools/swords reset

I generated a new world in MC 16w39c and I grabbed a sword. It didn't have an attack cooldown. I tested it on other tools and they also had no attack cooldown. When I saved and quit and opened the world back again, the cooldowns did work. I don't know why and how it happened, but there must be some bug causing this since I didn't do anything else in that world yet.