I am not sure if it can be replicated, but I was running the Hard Hydra Clan Boss and one of the heads ate my leader Siphi when it was her turn. I got the percentage of its HP buff cleared (visibly anyways), and it should have spit her our like normal, but it didn't, even though it appeared that the hp bar no longer had any of the green amount. Keep in mind that I did make several hits on the head (both regular and hp burn hits) after the bar had already been removed before making this video so there had to be quite a bit left.

As you can see in the video, a few more shots finally did it so there appears to be a visual problem with how much HP is left when trying to remove that buff. I suspect that maybe there is some math somwhere that is not being taken into account for that green bar. One thing that does spring to mind is Seeker currently has a 2 Star "Curelty" blessing set which will reduce the defence by 1% per hit each round, up to 20% for bosses. That is the only blessing I have right now for that team that I think could be part of the cause at least. If more info is needed like masteries or gear, I can provide it.