The bug

Turtles go upwards when you feed them seagrass when they are in water. However, since 1.13-pre5, they fall down into the water upon almost being out of the water. Before 1.13-pre5, they did float in air while feeding them seagrass.

How to reproduce

1. Spawn a baby turtle near the surface of water
2. Feed the baby sea grass by holding down 'use' while aiming at the turtle
   → Keep aiming at the turtle, see how it goes upwards and falls down upon almost being out of the water