Okay, Valk's abilities all seem to work as expected. For now. I'm still having the death box problem, but since last season I'm only having it with some death boxes. And relatively few at that. I keep trying to notice if the problem boxes seem to belong to any particular legends, or spot any other factor that might account for the problem. The only thing that's "a maybe" is that it's possible my squadmates are looting the box before I do and that the things I'm trying to take are in fact already gone... just somehow mistakenly still appearing as available. Not sure. I'll know more as time goes on.
In the mean time, try other legends and see if any others have the same issues with their ultimate, or maybe even with tacticals. Who wants to play Wraith anyway? ;)