If a field is both required and dependent, it requires another field to meet certain criteria. If these criteria are met by editing the record within the list view of the module, and the resulting field isn't in the list view; you can save the record without populating the required field.
There are two options to prevent this from happening:
Add the dependent field into the list view so it will not allow the save.
Advise staff not to make edits via the list view but to go into the record itself.
If this happens with a dropdown field, the cause may be a dropdown entry with a blank label but a database value. The field appears blank but is actually saving with a value. To prevent this, add a true blank entry to the dropdown list.