Somtimes the enemy will have started an attack just as you placate, which'll hit you and bounce you right back out of hide.
Apparently it is bugged as the people over seas have found, check this link out it might enlighten you more Here.
One of the leaders of the Harlequins villain group and currently on defiant.
Your spine attack DoT poison can seriously mess with placate as well 
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Your spine attack DoT poison can seriously mess with placate as well
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I did think that, but i tried my other 2 stalkers and it was same thing with them. 
Yes some powers cause trouble on placate ... for my own, i have some placate or hide problem sometimes . when i use caltrops of my ninjutsu secondary which is DoT too, but normaly the goal is to make fear on my ennemies ^^ ... of course i think about respec and not take this power (which is working well generally, but can cause my death sometimes)insted of i realy like it when it works ...
last 2 days from missions to pvp ive used placate on my spines/wp and every npc or enemy still attacks me ?