Hit 50 this morning on my Paladin, decided to get my Ifrit glowstick. So jumped into Ifrit HM with the knowledge I'd be stunbotting.

And discovered i could only Stun two Eruptions then he goes immune with the third Bash onwards doing nothing.

Did they change something, or has it always been this way?
For the record, I've cleared the fight dozens of times over several classes, Including WAR, so i know not to stun plumes.
As far as i knew, a PLD stunning only Eruptions could do so indefinitely. Was i wrong?