The rectangle lights ignore the specular value of lit materials and will apply a specular highlight to them regardless of the value. you can compare the result against a standard point light and you will not see a specular highlight for those.
Create an Empty Scene
Create a new material and connect a constant 0 value to Base Color, Metallic,Specular, Roughness,Anisotropy,Emissive Color.
Add sphere mesh to scene and apply material.
Add a rectangle light.
Observe specular reflection on sphere surface.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-130362 in the post.