Lumen screen tracing produces artifacts in a simple scene.
As shown below, the ground has a low roughness value (~0.2). When the screen tracing is enabled and the camera is facing the ground, Lumen gives white color under the horizontal bar of the “T” shape, as highlighted by the red rectangle. When the screen tracing is disabled (r.Lumen.Reflections.ScreenTraces 0), the artifacts disappear. In addition, when the camera is not facing the shinny ground, the artifacts are greatly reduced.
For more details, see this post: Lumen GI and reflection produces weird visual artifacts