When using 3D models that have been exported, converted and scaled from engineering CAD, it is nearly impossible to avoid having N-gons, flipped vertices/normals (non-manifold geometry), hidden faces and possibly many other unexpected topology anomalies.

CAD in its native form, be it NX, Catia or AutoCad are usually in a NURBS state (much like vector graphics where scaling does not affect the visual quality). It is not until the NURBS information is converted to polygons during export that these non-manifold problems arise.

A 3D model that has been mutated by this problem will cause anyone trying to UV map (or render) a model in this condition a variety of unfolding issues, surface irregularities and a seriously big headache.

For companies that I worked for as a 3D generalist (RPS Raytheon), I was not only responsible for acquiring the data, but also responsible for cleaning (prepping) the data before I animated and rendered it. Standalone applications such as Northen Hemisphere's Deep Exploration (now retired) allowed us to reduce the size of the CAD we were using (by reducing the face count) while doing its best to retain the original shape and look of the object. This reduction made the model file size smaller (and much more manageable) but over-reduction could drastically reduce face count to the point of ruining the final look. It is during this time of reduction and re-tessellation that can cause non-manifold issues to arise.

Today, Maya 2020+ (and many other 3D programs) all have much better tools for reducing and fixing this issue. During my time at Nissan automotive (Virtual Garage), RTT's "Deltagen" software was available to us as our go-to tool for data prep. Although I worked in Data Pull, I did clean my share of one-off parts.