Your Treasure Hunter green CP star affects only Overland (zone, delves, public dungeons, dolmens) Treasure Chests
When you encounter 4-man group dungeons or trials - the best you get from Treasure Chests in said instances are blue gear if you are on normal difficulty, and purple if you are on veteran. So group dungeons have another rules, unaffected by the green CP "Treasure Hunter" star
edit: for clarification: you can not get purple set item from 4-man Treasure Chest of the Master Difficulty (of from Advanced Difficulty if your CP star is slotted) if you have entered it on Normal difficulty, simply because purple items are not there in this mode. Items quality in drop tables in Group Dungeons or Trials is dictated by Normal/Veteran selection upon entering.