The Cascading Select custom field includes two drop-down lists. Currently, in 'required' mode, only the selection in the first drop-down is validated. However, this may not be sufficient in some cases. There should be an additional configuration option that allows administrators to specify whether the 'required' validation applies only to the parent option, or to both the parent and child options.

An example support case requesting that functionality: https://getsupport.atlassian.com/browse/PCS-342661