If you have facet of protection active (i.e. enough enemies around you to activate the buff), filaments will not give devour on rift cast or on refresh (stepping out and back in) until there are not enough enemies around to activate it. Also, spirit of the filaments is currently giving its ability regen on devour kill to spirit of the necrotic. I know it was listed as having necrotic's perk before the last dungeon, but it was removed from the known issues page while not being patched.