Dragonslayer and Mage Slayer wont trigger

[Stealthkibbler]

Trying to get the achievement for having Kingslayer, Mage Slayer and Dragon slayer, kingslayer will trigger on first kill but i cant get the other two to trigger at all, even after 50 dragons or 50+ mages.

[teddya78 (FR)]

however, no matter how hard I try to get units to kill mages and dragons, they never get the mage/dragon-killing trait.