Was a solution to this issue ever identified? I am having a similar problem with trying to slice a rectangular box with only five (5) sides (i.e. no top) and it is suppose to be empty inside. In the PREPARE stage it is displayed properly (except that the bottom is displayed in RED but when i slice it the inside of the object is infilled and a top is placed on the box. Below are the specs of my process:
Object created with TurboCAD Deluxe v2016 and exported into an .stl file
Object is opened in Cura 4.1 (had the same problem with Cura 4.0) and rotate 90 degrees so that the bottom of the box is flat on the plate
Review the object in the PREPARE stage to confirm that it is correct
Slice the object and then review it in the PREVIEW stage, Layer View - shows it as infilled with a top.
For reference i have included the .stl file of the object as well as the gcode produced by Cura.
NOTE: I use an Anycubic i3 Mega printer for which my Cura has been set up for.
Thank you.
Holder-20190704.STL
Holder-20190704.gcode