Try it yourself: Open a form in edit / design mode. Hold the Ctrl key and use your mouse to select a field. Right click to bring up popup list, then left click on Control Properties. Scroll down in the dialog to Border. Change the drop-down from Flat to 3D. Close the Control Properties dialog. The field now displays as 3D.
Save the form. Save the Database. When next opened, your changes will be gone. The field has returned to Flat.
Note: Changes like moving a field to another location on the form save normally.
Version: 7.5.4.2 (X86_64)
https://git.libreoffice.org/core/+log/36ccfdc35048b057fd9854c757a8b67ec53977b6