GCode output starts missing paths when the path touches the board edges

[SteveEggo-SpecialAgent]

My original intention was to remove all of the copper from a board except for the tracks and pads. Using Eagle with PCB-GCODE V3.6.6 the resultant GCode file does not achieve this. To reproduce the problem:
Board 75 x 50mm, 25th track 25mm in from each edge. GCode setup shown, using a 1mm cutter

Preview of tool path. Note the big blank area where copper is not removed. The tool path should cover all of the board.

Chilipeppr display showing the same information (just in case the viewer was not telling the truth)

Note this information was supplied on Groups.io 02/05/24, but no one has answered it.