Olmor sorry, but I debunked this. Or at least as of Version 25 this is no longer true 
I just did a check of this, in the interest of checking whether the order of buffs and ailments matters.
I found out that DoT, is always calculated before a HoT, regardless of the order the specials were fired.
Scenario 1: Poison DoT casted first, then HoT casted after
Result – Lianna takes Poison damage first, then HoT heals her
Scenario 2: Gadeirus HoT casted first, then Poison DoT
Result – same as Scenario 1 – Note that Lianna is taking damage before the HoT activates, despite the fact that the HoT was casted first.
@_John_Doe now we know 