https://github.com/EpicGames/UnrealEngine/blob/master/Engine/Shaders/Private/PlanarReflectionShaders.usf
After looking the code it’s obvious that prefilter only blurs toward vertical axis. To get really blurred planar reflections you probably want to set screen percentage down and add horizontal blur after vertical one. If you don’t want to modify engine code you can do horizontal blur when sampling for planar reflections.
Replace this line with code that do multiple samples and scatter reads in x axis. https://github.com/EpicGames/UnrealEngine/blob/master/Engine/Shaders/Private/PlanarReflectionShared.ush#L91