No error message displayed or shown in the logs. The field just always shows as blank as if no item selected. See the image below:

This issue is due to a defect within eDP 10.1.2 only.
This issue is fixed in the following release(s), available in the Download Center at https://www.veritas.com/content/support/en_US/downloads:
- eDiscovery 10.1.3
Workaround:
Even though the case looks as if not selected, the Save button can still be selected and the case will be associated with that legal hold. This can be seen in the All Legal Holds view showing the Case column like the following:
