The gantry is not perpendicular to the base, it tilts towards the back (on the side of the Z-axis motor).
The inspection measures the perpendicularity of the Z-axis material to the platform, not the perpendicularity of the Z-axis material to the surface of the base.
3.1 Measure the perpendicularity of the profile and the platform using a right angle ruler.
3.2 Remove the Z-axis profile, check if the profile cross-section is uneven.
After checking the above steps, if the problem still cannot be solved, please contact after-sales service cs@creality.com