Boolean Modifier adds unexpected vertices

In this file: Headboard Inset - Forum Booloean Question.blend (946.0 KB) the object “Boolean end left” is used as a boolean object to trim the ends off the “Inset” object. It does do this as expected as indicated by toggling the Boolean Modifier’s Realtime icon.

What is not expected is the boolean modifier adding vertices to the “Inset” object that have been copied from “Boolean end left” object. These new vertices remain after the boolean modifier is applied. The addition of these vertices is undesirable, as it requires additional editing to remove. Why are these vertices added? How can the addition of these vertices be avoided? Is this a Blender bug?

This file was created in Blender 2.90

Flipped normals and “fast boolean” that does not support co-planar faces