Dolphins seem to despawn (not die).

My experience: I created a pool to dolphins with air and water. I name tagged them. After I walked away (like 50-100 blocks) they despawn and left nothing behind them. Also if you stay with them and time passes by like 5 (irl) minutes, they despawn. They leave no exp or items. They just disappear. It doesn't matter if they are in the sea, pool, the biome doesn't matter, neither does the weather or if there are one or multiple dolphins.