When the status of a work item is marked complete, clients will no longer be able to access any Client Tasks assigned to them within that work item.
If you need your clients to regain access, updated the status from completed to something else. You can then uncheck one of the tasks and resend the client tasks. Or, simply add a comment and your client will receive an email notification.
30 days after the initial Client Task email is sent, the Client Tasks link will expire. If you've set up reminder emails (recommended), the link for your Client Tasks is extended for 60 days from when the reminder email is sent.
Best practice tip for UK firms and annual work
As a workaround, consider setting up your first Client Task email without reminders. Then, set up a second set of client task emails to auto-send later in the year with reminders.