I can't read the title field and am on my phone.. so I hope I typed it correctly!
Dark Imbument doesnt list a cooldown in its description, but it has one. this makes it only work on one hand when dual wielding and it doesn't go off on each attack when using haste boost. I think it goes off on each attack when fighting with a two hander, but not positive due to no useable combat log and no usable test targets to use as anything that lives long enough to get a good number of sample hits will kill me solo, so I test in groups or on targets too weak to test effectively.
with how fast attacks went by when using haste boost it was too hard to check how often the damage showed up, but definitely was not every other hit when dual wielding.
when I had tried it before with a repeating crossbow I had thought it was just broken and didn't work correctly with them, but after trying it with an assassin using vistani I found it is due to some sort of cooldown on the damage that isn't listed in the ability.
I don't know if it is a bug or just something they forgot to put in the description, but figured I would put it on the forums so people can know about it.