If a model has parts of the main mesh split into proxies, those proxies do not get displayed on the item preview widget, this can be seen with any mod that uses that widget like the example below with a vanilla vehicle that has parts of the interior split into a separate proxy due the old vertex normal limitations.
The problem with this is that since vehicles have attachments, and those are proxies, those attachments do not get shown either, that behaviour affects only vehicles and not other items like weapons or any basic item, this limits the type of customization and preview that can be done to a vehicle in something for example like a trader mod or anything that is used to customize and preview changes on said vehicle in real time.