Night Sister res through Death Trooper death mark

There’s no bug. Read the Ability again.

It specifically states that it cannot revive if killed by TERMINATE.

It does not state that it cannot be revived if there is DEATHMARK debuff.

If you managed to land the death mark debuff, it means that you didn’t manage to finish off the NS with Terminate, so they can and will revive.

The in game description for deathmark disagrees with you.

Read it again. It says can't revive, not can't be revived. 

That's possibly the dumbest reading of that text I can imagine, and a definite butchering of the plain meaning of the words. If you can't revive (restoring to life and/or consciousness), then you can't be revived (having been restored to life and/or consciousness).

I'd also note that on April 25th, there was an update that had the following bug fix:

• Deathmark will now correctly prevent units with auto-revive mechanic from coming back to battle after being defeated.

 See: https://forums.galaxy-of-heroes.starwars.ea.com/discussion/167687/content-update-4-25-2018#latest

Ability is WAI as per the full text

 However the reminder text as seen in the screenshot above should definitely be edited for accuracy/clarity.

Except it's not actually working as per the full text. Just because the text of the ability doesn't spell out what the effects are of the deathmark debuff, it doesn't exclude those effects. Deathmark, in every location it exists, prevents resurrection -- from p1 of the pit raid through grevious' deathmark in the ground war assault battle mythic tier. It's what the debuff does. 

Also, the reason the ability needs to say that terminate prevents revives is because 1. if it's the first kill it still does it; and 2. if it's a subsequent kill, you want terminate to be at least as effective as the deathmark that would've been applied.

Usually raid and event bosses get suped up versions of abilities like this, but I do agree it can create confusion with the character versions.