The Mirror Network group is not random, it’s tied to whatever Per Battle World Quest is currently up in Revendreth.
You can even run this command in game to see what Mirror Network group is active (I had it in a macro when I was still farming this):
/run m={61879,61883,61885,61886} for g,n in pairs(m) do C_QuestLog.RemoveWorldQuestWatch(n) a=C_QuestLog.AddWorldQuestWatch(n) c=C_QuestLog.IsQuestFlaggedCompleted(n) if a or c then print("\124cff00FE00Mirror Group " … g … " active\124r") end end