What you were expecting to happen, and what actually happened?
Kerberos fired at my fully traited Behemoth. It had Impervious, so in theory it was immune to devour, but … It was devoured 
What are the steps to make it happen again?
Fire a Kerberos at an Impervious Behemoth, and succeed on the 40% devour chance.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!
Guild Wars paragon battle, Pchancete vs Phyxius. 2017/09/02 at 11:20 UTC+2 aprox. Unfortunately, on the screenshot you can’t appreciate that the devoured troop was a Behemoth.
