We have a DGN file containing the two complex solids that appear to have very similar characteristics, yet one solid displays "cleanly" as expected in a cached 2D drawing while the other leaves the faceting visible. These solids were modeled by placing a solid block, dropping it to surfaces, meshing the top surface to follow the contours of a Point Cloud, then recombining the surfaces into a solid.
The core issue appears to be with how the Remove Smooth Edges option is processing the Cached view. When that option is disabled both solids will display the same level of faceting:
In order to improve the results while leaving the Remove Smooth Edges enabled, you can change the hidden line method from the default Exact to Fast instead which may allow both solids to display without the faceted isolines:
You can also try converting the "problem" solid into a mesh and then back into a solid. This may also improve the results without needing to adjust the hidden line accuracy.
Note that the method used to create these solids may be a factor in how they behave in post-processes such as drawing generation, since some of the meshes used appear to have had minor gaps and/or overlapping vertices. In these cases it may be best to use the available mesh auditing and repair tools during the solids creation process to help correct these anomalies.