Plagued crits is a weapon perk that reduces healing when you crit a target below 50% health. It lasts 6 seconds and it has a 10 cool down timer. Once the perk is activated the 10 second cool down starts and you cannot apply it to the same target OR a new target. At least that’s the way it seems. Maybe it’s just bugged.
Each target should have their own individual cool down. You should be able to apply plagued crits to each new target.
Additionally, the perk is surprisingly underwhelming. It could probably use a buff like 50% healing reduction (or even 100%) instead of 25%. Healers seem to be able to heal through it (assuming it works at all). Alternatively, just remove the cooldown. Allow people to reapply the debuff.