You have a site that contains lists with columns that are connected to a Managed Metadata term store. You export that site and import it as a new site. When you add a new document to the list in the new site and specify a value for the Managed Metadata column, that value is not retained. Instead the column remains blank.
This issue happens when any of the below is true:
The site was exported without user security information.
The site was imported without user security information.
A template name was specified when creating the new site before import.
For example: New-SPSite-Url <New SiteUrl> -owneralias <alias> -Template "STS#0"
Ensure to include user security information in export and import operation AND do not specify a template when creating the import site.