Strange walls/faces when importing self made model from Blender to Cura 3
It is quite common for modelling operations to result in 'non manifold geometry', meaning that some of the faces intersect or are not perfectly joined.

Although there is nothing obvious in your model, you can check in blender by going into edit mode, unselect all, selecting nodes, and 'select all by attribute/non manifold' Ctl-Alt-Shift-M (if I remember right).

Usually this happens with boolean operations where you merge the same object twice (i.e. union for the hole, then difference with the cut part). You would need to apply the boolean to see the non-manifold result.

Any of the standard mesh fixing tools will attempt to clean up your exported STL, and you could then import the model back into blender (but then it is a one way process, and you can't make updates to the base model).