User Mentions on pages are broken after space or site import from Cloud

Issue Summary

When a page with user mentions is migrated from Confluence Cloud into Confluence Server/DC using either site or space migration, the mentions display "Broken Link"

For a cloud to cloud migration via Confluence DC, this results in mentions showing "Unlicensed User" in the destination cloud site

Steps to Reproduce

1. Create a page in Confluence Cloud with one or more @ user mentions
2. Export the space or site from Confluence Cloud
3. Import the space or site into Confluence Server DC
4. Observe the Broken Link message in place of the user mentions. Within the page source the <ri:user> element is missing a user identifier attribute.

Expected Results

The expected result is that the @ user mention is displayed with the proper user in Confluence Server/DC.

<p><ac:link><ri:user ri:userkey="2cb28085823708f50182370d3c880001" /></ac:link>&nbsp;</p> 

Actual Results

is displayed

The below element is what is actually created in Confluence Server/DC:

<p><ac:link><ri:user /></ac:link>&nbsp;</p> 

Workaround

For cloud to cloud migrations: The workaround is to use the space export/import directly from cloud to cloud.

For cloud to server migrations: Currently there is no known workaround for this behavior. A workaround will be added here when available