When adding an object field to Display in default lists and hovercards, it does not get added as a column.
This can happen if the user customizes the object records view. For example, if the user resizes columns, or removes or adds columns, it is considered custom.
The user has to manually add the field as a column using the Action menu. This is the Action menu located at the top-right when viewing object records:
Vault Help Documentation: Configuring Vault Objects