It looks like there should be 5 missions.
I don't think what you're describing is a bug.
After completing all 3 Tasks, when we get more of that Tasks? Weekly? I'm wondering, raising the Number of Party Members to 25 needs 200 Points (atleast), and some of the existing Kabals already have 25 Members. So how they could reach 200 Points with three Missions only?
Only three at start, you can unlock other 2 by cabal tech