Greyed/disabled author link/merge boxes

The "link" and "merge" tick boxes to the right of any selected author name on any given "edit author information" page are greyed out or disabled. Only "retain as 'alternate form'" is enabled and ticked (see attachment)
This was no the case a few weeks ago, but unfortunately I can't say exactly when it began to happen nor what we did (if anything) that made it happen.