Heightmap

[BasementSam]

I want to mill a PCB. I created a heightmap:
0.00;0.00;31.48;36.56
3;3;-2.00;1.00
0;3;3
0;0.11999999999999922;0.13999999999999968
0.08800000000000008;0.21999999999999975;0.21999999999999975
0.11999999999999922;0.22499999999999964;0.2469999999999999

However when I select Use heightmap my gcode changes from
G00 Z5.0000
G01 Z-0.1500 F254.00
…
G00 X17.6530 Y24.9266
G01 Z-0.1500 F254.00 (for example)

to
G00X0.000Y0.000Z4.999
G01F254.00X0.000Y0.000Z-0.151
G01F508.00X1.105Y1.105Z-0.142
...
G01F254.00X17.653Y24.927Z0.107

That is, all the z coordinates after the first two which ought to be negative for the cuts, are positive, and the spindle remains above the board.

What am I doing wrong? If this is not the right forum to ask this question, can you point me to a more suitable one?
Tech details: Windows 10, Candle2 ver 2.3