How to reproduce:
1) Create Backup from Cloud, using Moving to Confluence Server option.
2) import backup to Confluence DC
Result:
<ac:link><ri:user ri:userkey="8a7f80897530d198017531030f6a000c" /></ac:link>
ri:userkey="8a7f80897530d198017531030f6a000c" just wiped and not present even in Database.
So All mehtion link in all spaces and all pages are broken
