Please share the code of overridden property and any customization on that (if any). I tested overriding a property (Category) and just copied original get/set code and it was working.

Without seeing your code, it's hard to pinpoint what is the error, but making a property required may also be done like this:

[EditorDescriptorRegistration(TargetType = typeof(CategoryList))]
public class MakeCategoryRequiredEditorDescriptor : EditorDescriptor
{
    public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
    {
        base.ModifyMetadata(metadata, attributes);
        if (metadata.PropertyName == "icategorizable_category")
        {
            metadata.IsRequired = true;
        }
    }
}