Hypothermia - Persistence Support does not increase curse duration

Currently the Hypothermia curse has Persistance as its recommended support link.

Hypothermia tooltip without support gems attached shows a duration of 6.4 Seconds.

Persistance should increase its duration by 40 % yet the tooltip still shows 6.4 Seconds.

I was wondering if this was only a tooltip bug or if the support gem is bugged.

Same problem with Unearth minions, their duration seems unaffected by Persistence support.