In ServiceDesk classification hierarchy data there is a classification with '&' in it. When this classification is selected, none of its sub-classifications can be selected.
Classification hierarchical data resides in the Hierarchy Data Service of Workflow:
Admin > Data > Hierarchy Data Service
Hierarchy Data item names are expected to contain '&' in encoded form as '&' to function properly. When item name contains '&', the EnsembleMenuSelect component responsible for showing the items fails to show it properly.
This will not happen when items are added from Hiererchy Data Service page. Such items, however, can be added when importing items in bulk.
To resolve the issue the item names must be changed to contain the expected '& instead of '&'.
The best way of doing that is in the Portal.