Anycubic Kossel Plus raises the Printhead around the corners of the bed too High if the print is too close to the corners
It is a common  problem with "delta"-style circular parallel kinematics machines.  It is called Bowling.  It can lead to the presumed Z=0 plane having a bowl-like shape.  The actual error shape can be more complex.  The job of "bed-leveling" is to make the realized z=0 plane to be identical with the bed.

If you can, use a multi-point (like 40+ points) interpolation.  It builds an error map, and adjusts every point based on that map.