This behavior has been observed in 4.17, 4.18, and 4.19 release versions.
In our game they player's hands are represented by glowing blue orbs and in planar reflection surfaces they aren't rendering correctly. I've whittled this down to additive materials showing up funny in planar reflections when they don't have an opaque backdrop.
Result: Sphere with Additive material does not render a reflection in the Planar Reflection actor unless mesh with Opaque material is behind it. Changing the material blend mode to Translucent produces the expected reflection.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-58558 in the post.