Herald of Plague never hindering

I've had herald of plague active for days and days, and it does spread poison to other enemies within the stated radius, but it has never hindered enemies, even though it states that it has a % chance to hinder enemies that the herald has spread poison to. I've taken some time to carefully look for the hindered modifier icon, as well as just looking to see speed reductions in monsters, but it's never happened

Can confirm this skill is bugged.

What's worse: Adding quality to the skill gem should increase the hinder probability by 20% but it still never happens.