The tg wiki states this:
Is this actually the case on the core tg version of the game?
Antagonist Assignment
Before jobs are assigned, the game will decide who the round antagonists are. A player who would normally have spawned as Security will spawn as a different job instead if they are selected as Antagonist.
This means you can have Security jobs on HIGH without having to worry about never getting antagonist roles.
Because on the derived version I usually play this sure as hell isn't the case and only applies to malf, wizard and nuke ops. When having security set at high you will NEVER get traitor, rev, cult, gang or DA (with a replacing job).