You have set the KUT field as mandatory, however you can save the document even with the mandatory field unfilled.
SAP Cloud for Customer
Reproducing the Issue
1. Go to Installed Base work center.
2. Open the Maintenance Plans.
3. Open any Maintenance plan or create a new one.
4. Go to Maintenance Items facet.
5. You can see that the Start Date field under preview pane is empty but marked as mandatory.
6. Change the document but do not fill the Start Date field.
7. Save the document.
8. The document is saved even with the field unfilled.
This is the system expected behavior.
It is not possible to set to any field as mandatory which is under preview pane using KUT.
This is implemented to the consistency of list data.
KUT, mandatory, unfilled, blank, field, save , KBA , AP-RC-UIF-FLX , Page Layout & Key User Configuration , How To