What if the group id was, let’s say 1020, and was pre-existing upon migration on targeted machine, assigned to some other group name? What if the group name merely was already there upon migration? I don’t know exactly how these checks happen upon migration but it’s easy to mess it upon such clashes. For the future, @adamjedgar in case it’s Virtualmin bug, could you remember what you were trying to import (in terms of user and groups) and whether a group with the same name was present on targeted system prior to migration? I would also did the same checks for group id.
Have you tried simply running:
ls -lsa /home/migrated_virtual_server
Does everything look correctly out there? Are there only group ids set for the owner, by any chance?