Can't seal dispel

Description of the bug: I'm a f/p mage and sharks can cast dispel.
So I tried to cast seal on them beforehand, so they don't dispel me.
It appears they can still cast dispel and cleanse themselves of Seal.
What is supposed to happen?: They shouldn't be able to do anything but aimlessly wander when sealed for 18 whole seconds.
Where did you find and/or notice this bug?:
A step-by-step guide to re-producing this bug (optional):
Cast seal on a shark.
Notice the "..." symbol above its head to confirm that it is, indeed, sealed.
Notice it no longer being sealed maybe 3 seconds later or so.

Not 100% sure if it's a bug or intended. But if it's intended, it's very counterintuitive and plz change it. They are already impossible to farm as fire mage.

 

Yeah, I was in a party with a chief bandit. I think this occurred even if they afk at the top.
I can test again solo.