What I expected to happen was...:
The polar bear would swim
What actually happened was...:
The polar bear walked on top of the water
Steps to Reproduce:
1.Go in a iceburg filled part of the ocean
2.Find a polar bear
3.Watch it swim
P.S. the polar bear started to swim afterwards, so it may be hard to recreate
Please check if that's still an issue in 1.13.1.