To fix this, switch from screen space reflection to planar reflection for the floor.
The reflection of the billboard looks weird in Unreal, what do I do?
FAQ Content Creation Composer 3D Scene Editing Unreal
Due to Unreal Engine’s limitations screen space reflection doesn’t work well with transparent objects (like Billboards).
Planar reflections are great for planar surfaces, but it’s very demanding on the GPU and doesn’t work for all kinds of reflections. One solution would be to use a linked copy of the video billboard that uses a masked material right “behind” the translucent material. To keep the soft edges on the translucent material, the masked material would need to use an eroded alpha matte. Erode in UE4 is already available in Composure, so that can probably be used for this as well.