If you have a Coral Plant on a Coral Block so that the plant is in the top block of water. If you break the plant the water disappears. Also in the same situation you can't pick up the water in the Coral Plant block with a bucket.

Edit: You can't pick up the water on a Coral Plant when it is surrounded by water either. Possibly related to MC-127017.

Edit 2: This happens with Coral Fans as well.