This is technically not a bug, as Bonewidow is by definition not Voidrig. HOWEVER! If the bounty objective stipulates killing Voidrigs, but only Bonewidow spawns for 20 minutes(as i just experienced mere minutes ago), the mission cannot be completed until the player waits for Voidrigs to finally spawn. This negatively affects the player as if it were a bug, thus i felt it made sense to report here. Perhaps change the objective to "Kill 2 Voidrigs or Bonewidows"?