I brought the stl file into my meshmixer and used Edit, Generate Face Groups. Accepting the defaults results in different colors. The rectangle in question then shows up clearly as being at least one layer higher (or maybe lower) than the rest of the part. Meshmixer sees it as an entity of its own because of the abrupt change in elevation.

I also used Edit, Plane cut, rotated the cutting plane to vertical referenced to the major face of the part and did not remesh/fill. The resulting slice shows clearly there is an internal double wall to the construction rectangle.

These are design flaws that some slicers may ignore or otherwise repair, but sometimes not.

I wish I was more skilled in meshmixer, as I would expect one could collect these components and merge them into a single entity, ridding the part of the glitchy sections.

I just tried something else.

Using Edit, generate face groups, then use Separate Shells to create two independent objects.

By playing around with scaling, I was able to determine that the inner rectangular structure is larger than the enclosing part. I was able to scale the inside portion to 0.995 uniform (although I should have left Y alone) and make the double wall disappear, but only to the eye. I'm unable to tell if the two walls are close enough together to call a single wall.

Unfortunately Meshmixer's skill with Boolean Union is miserable. It distorts the shapes beyond use.

I exported the individual parts, then used OpenSCAD Import() to see what I could do with that program. It previews fine (F5) but fails as expected with the rendering. I don't understand everything I'd like to in OpenSCAD, either. If I was more capable in Blender, I could probably bring either the main file into Blender and fix the double walls, or bring the two parts into Blender and properly merge them.

Since I have all these wonderful tools with which to manipulate models, I popped the stl into Fusion 360. It really shows clearly where the double wall falls. From what little I know of F360, the next step is to perform a mesh to Brep action. At this point my F360 goes belly up. That's not a good sign at all!