The bug:
Mobs that can swim in water like fishes and dolphins stop swim after spawned naturally or spawned by player.
Here is a video demonstration: https://drive.google.com/open?id=10WwSl8Cr9TSmhCUTUiRoxygN9xs32cC3
MCPE-47868
Swimming mobs stop swim in the water
Resolved
Fixed
2
(Unassigned)
206976
Confirmed
Windows
Windows 10
incorrect, mob-behavior, swimming
1.13.0.1 Beta, 1.13.0.2 Beta
1.13.0.5 Beta
Retrieved
Where's the same bug, but for Java edition?