When my spawn chunks (which are set to 1000/1000) on my server are outside the worldboarder, they are unloading, and can be consistantly manually unloaded using the /save-all command. I do not know if this is a bug or not, but in my sense spawn chunks should be loaded always, regardless of worldboarder.
Only tested in snapshot 18w15a on a vanilla server.
Is this still an issue in the most recent versions (currently that is 1.13.2, or the latest snapshot 18w46a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.