This problem has cropped up in these forums a few times.
It is the Level Of Detail settings (LOD).
They may be controlled in the object settings or the object may be set to use the Model View Options (MVO) settings.
To me it is a bug that the object preview does not show the object as it has been set when the LOD is controlled by MVO.
It does seem to show correctly if the LOD is set in the object itself (i.e. not to "by MVO').
Or there should be a warning to say what you see in the preview is not necessarily what you will see in the model.
You will want to check the settings on your MVO.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10