In a mirror workspace, a link column may offer no value to select from the record. This is not a bug: the link view to use in that workspace has simply not been set.
The cause
A link column displays records from another table through a view of that table. As long as no view is selected, the column has nothing to offer in the mirror workspace, and the list appears empty.
The fix: select the link view before sharing
Before sharing a view into a mirror workspace, choose the link view that applies to that workspace, on every link column concerned.
This setting also has a security effect: the view you select determines which rows and which columns of the linked table are visible from the record. By pointing to a filtered and restricted view, you only expose the data that workspace actually needs.
The reverse is also true. A link column pointing to the table with no view selected can give access, from the record, to data you had carefully hidden in the shared view. Check link columns just as you check the columns of the view itself.
Go further
Create and use a mirror view
The full setup between two workspaces.
Go further
Selection filters on a link column
Control what the link column offers for selection.
Or continue with
Link view types
The display modes available for a link column.