I exported my all my documents looking to edit a few of them with a script and re-upload with changes.
When I export my blog post documents, I have a group field labeled tags, I think these tags get overridden by the built in Prismic tags when exported. When I import the document back in the custom tags are missing off the document.
I would need some extra clarification and information to be able to help.
If you mean that you lose the link to the custom tags when importing the new documents, then it is expected as the import feature doesn't maintain links.
But it seems that you mean that the field tag is missing from your imported document.
Can you also share with us your repository name (in a private message if necessary)?
Update: The cause of this error is because Content Relationship fields and all Links to internal documents aren't supported in Import / Export. You need to re-add them after the import is complete.