It might be a side effect of the fact that refraction in Unreal 4 is handles using a post process effect. So it might be a side effect of that. Maybe someone from Epic could clarify on why this would be happening, or maybe it is an actual bug.
For now you could use a custom distortion, which is easy to implement:
It is basically the same way most engines do it, accompanied by the standard side effect that everything in front of the water is also part of the distortion. This is because we use the currently rendered scene as a distortion texture. This is a very basic implementation, you could always improve it.