importing a stl bug

[wormpower32]

i have been trying to import and edit a stl file.

when ever i import the stl and generate face groups. somehow the object when i export as stl loses 2 layers in the middle of the object.

the only 3 things i did were

insert Mesh

generate face groups, select accurate, tolerance 0.001 mm

export stl

opah_extruder_brace_v0.4 is the original file

Moarstrudder Brace v4 is exported stl

when you look at the latter in layer view in cura. there are 2 whole layers missing layers.

what am i doing wrong?