In the community balance mod, halberd heavy sweep was given the ability to cleave armor and, to do so effectively, was given the Heavy Linesman mass modifier.
In the balance beta halberd got all the changes the community mod had, including the Heavy Linesman mass modifier. But it is missing its armor cleave.
It must be unintentional, as Heavy Linesman is identical to Linesman as a mass modifier against infantry targets. Heavy Linesman is completely pointless on an attack that can’t cleave through armor/mauler/monster.
Either it not being able to be cleave armor is a bug, or it getting heavy linesman was an oversight and should be changed to regular linesman.