MD triggering before buff expires

"Your mystic champions accrue x% power each time an enemy beneficial effect expires" that's the mastery description. So why does it trigger as soon as you get a buff? I was fighting a juggernaut in aw on node 40 with Electra...please look into this
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But it shouldn't trigger with the first dexterity buff of the fight, which is what happened. Hence why I made my post