Possibly, Relations between Notes that are visualized in Sheets, were not created on the Sheets that show them. These Relations are marked by an asterisk (*).
When you export particular Notes and Sheets, Relations between Notes that were created on other Sheets nòt in your selection, are nòt exported. After importing this export, Relations are missing.
If you want these Relations to be included in your export files, make sure that the Sheets where they were created, are included in your search results, from where the export is enabled. On the Sheets where the Relations were created, they are not marked by an asterisk.
You can also delete the Relation between the Notes from any Sheet where it is used, including the one with an asterisk, and create a new one instead on the Sheet you wish to export, so that it does not carry an asterisk on that particular Sheet.