Spawn chunks unloaded outside worldborder

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.