The "white" PCB outline is created automatically by FAB 3000. It was primarily meant when loading ODB++ and IPC-2581 files because they "always have clean defined borders". When loading Gerber files you can get "anything" to represent the border (from filled polygons, hundreds of separate arcs/line segments, etc.). The Automated White Border outline is only created when FAB 3000 can clearly detect a border. Here's the rules for a border, https://support.numericalinnovations.com/support/discussions/topics/14000006947
Note: Once a clearly defined border is created FAB 3000 will attempt to generate it in the background. If that border generation is causing any angst, you can disable it (we just added that border outline display recently - previously people would just make the Border Layer visible (i.e turn on) to view in the outline in Jobs & Panels, etc). https://support.numericalinnovations.com/support/discussions/topics/14000018807
The slot is there when viewing the layer "milling.gbr". The reason it is not showing up on the FAB 3000 generate "automated profile" is simply because the slot contains 74 individual line segments. If you wish to have FAB 3000 display the white profile outline, that feature will need to be a closed
Again there is no data missing, however when working with Gerber files the profile outline data is rarely clean (i.e FAB requires a closed boundary on a layer type defined as "Border") - so FAB 3000 will not display the slot as a white border outline until it is closed. Here's my quick tutorial showing me display the slot individual objects, https://numericalsoftware-update.s3.amazonaws.com/tutorials/2022-06-03_20-07-52-profile_display.mp4
Note: When working with Gerber files, it may be better to just uncheck the "Profile Display" (menu: View | Outline/Profile) to prevent any uncertainties because it's only there for display purposes (or just verify all border layer objects are closed).
Note: We will look to see if there's any way we can perhaps have the Border outline feature - try to join the Open Slots (however with Gerber data it can get messy trying to automate the border detection.
Thanks again, and please let me know if you have any other questions.