Does FreeCAD produce a wrong stl for slicer software?

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I designed a basic structure with FreeCAD. For the base, I started with a 40×40 mm square:

Screenshot of FreeCAD showing a 40x40 mm square

And I extruded it with 6.5 mm:

Screenshot of FreeCAD's Pad parameters dialog box with a 'Length' of 6.5 mm

Then, I added a circle sketch with a radius of 19 mm:

Screenshot of a FreeCAD sketch showing a circle with a radius of 19 mm

And applied 5.5 mm pocket:

Screenshot of FreeCAD's Pocket parameters dialog box with a 'Length' of 5.5 mm

The end result looks good:

Screenshot of a square base with a circular recess 3D model in FreeCAD

However, any slicer I know will produce a strange shape around the middle of the sides both inside and outside:

Screenshot of a sliced model with unknown artifacts at the tangent edges of the circle and square sides

There is a 1 mm thickness there, so I don’t understand why are those shapes presented.

I’m looking for:

  1. The reason for those failed shapes
  2. The solution to eliminate them (preferably without resizing either the square or the circle)

For @Mołot’s request, I uploaded the stl here: filetransfer.io/data-package/ohThffuy

(It’s 25k, so not suitable pasting here).

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