Whenever a bounty requiring Vex bosses be slain has been picked up, I try to complete it in The Garden World. However, beating this boss(obviously Vex) does not grant progress towards the bounty. The only Strike I can complete this on to my knowledge, as a New Light player, is Pyramidion. Having gotten the bounty to do this objective twice and both times not gaining progress vs Dendron, Root Mind, I believe this is a bug.