Did a quick test, it's working as of 19.4. Take a look at the box on the top right of the screen, the debuff that pops into it is what's on the zombie and it's the slow debuff. Did some moving around with the zombie not having and having the debuff for comparison.
Thank you so much for checking that. It appears to be a very short (de)buff, so I was probably just missing it, but you prove here that it is indeed working.