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.
A cause of this if it is a dropdown field is when you have a dropdown entry that has a blank label but a database value. It may appear as blank, but it is actual fact is saving with a value. You would need to add a true blank line to the dropdown list to prevent this.