Have you turned off the Shield's AI Enhancements upscaling thing? That adds sharpness and can cause artifacts.
Also, please elaborate on what you mean by "motion artifacts"? Like what, scan lines, ghosting, blurr...etc.
What is the source of the video that has issues, OTA, TVE, local content?
Check to make sure the Video output setting on the Shield is set to the correct resolution and frame rate for your TV.
And also that any Motion enhancements on the TV are disabled, as that can conflict with certain content.