I've been spending lair keys when running pathways and only getting engrams & some glimmer from the chest each time, this was a known issue a while back to do with being first to the chest but it was reported as having been fixed, however I am still not getting the expected rewards from a completion. I can't be 100% certain if I'm first on any given occasion, but I am pretty sure I have been first to open it on several runs and still received nothing.