Ok i could reproduce OP’s problem and will add it into Bug Report.
When you are specced into Pyromancers and let them constantly auto attack the dummy, you can see that they do ignite stacks.
Once you put a point into any other node of Skeletal Mages skilltree they immediately lose their ignite chance and it will switch back to poison stacks.
@Shinku you probably leveled with Skeli Mages and specced into Pyromancers, you continued your progression and put more points into Skeli Mage tree while they were still alive.
So always make sure to recast your skelis whenever you put more points into the passives or skilltree because of current Snapshotting problems in the game.
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