Hey there,

Drop rates for any ninja in game, be it Chase Mission, Mission Box, Treasure Draw, etc., it's random.
It is something that may happen in large quantities for some players, or not happen at all for others.

On my own example I can say, I have never seen Tendo in chase mission, but I got him once from Mission Boxes. Other than him I got GNW Kiba once, but no other ninjas from those boxes ever. There are people on my server that also got Tendo from boxes, that got Hashirama from boxes, or Tobirama, and some that are 1-2 fragments away from Tendo. It just varies from person to person, and is not something that is possible to really predict or "fix", as it's not a bug.