If mandatory field is needed in two different situations, the Set Field Mandatory action needs to be used twice in Workflow State.
1. Creating a new Record. Field is marked as mandatory if user selects value X in field Y.
2. Editing the same Record. Field is marked as mandatory if value X is already set in field Y.
There will be two Set Field Mandatory actions:
1. Trigger is 'After Field Edit'
2. Trigger is 'Before Load'. User can create a condition via VisualBuilder to cover correct behavior.