Systematic tilt on plane coordinate along depth axis

Hello. I am developing an AR measurement system in Unity using Vuforia Engine for a Windows Standalone build rather than UWP. My goal is to track a single Image Target moving on a perfectly flat physical table to precisely extract its spatial coordinates.

I have tested this setup using both a standard PC webcam and an external Canon DSLR camera with a 50mm lens. Using the 50mm lens significantly improved the overall tracking precision and almost completely eliminated the "bowl effect" spatial distortion caused by wide-angle lenses, but a systematic error persists. As I slide the target away from the center of the camera along the depth axis, the tracked height systematically drops or rises instead of remaining at zero. This creates a virtual tilted plane effect, although the tilt angle is noticeably smaller with the 50mm lens compared to the standard webcam.

Plane visualization made with MATLAB (webcam)

I tried changing the World Center Mode to both "Specific Target" and "Device". I also integrated the Vuforia Ground Plane feature using the Emulator PDF printed on the table to provide a stable anchor for the tracking. Despite giving the device tracker a solid reference, the systematic tilt along the depth axis remains present in the raw tracking data.
Is it a known problem of Vuforia and is there any fixes?