When using a Caster Frame sword such as Ill Omen with the trait Cold Steel, heavy attacks do not apply any Slow stacks to PvE combatants. The perk description states that powered sword attacks will apply Slow. Light attacks do correctly apply Slow, freezing a target in 3 light attacks. Hitting a combatant with 5+ heavy attacks does not freeze the same enemy.
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Same thing with vortex frames, it doesn't apply cold steel on the heavy. My guess is that it's intentional since both frames' heavy attacks hit multiple times in quick succession, and it may deal to much damage