Maces investigation mechanic reads as:
Whenever a Jedi Vanguard ally uses an ability on their turn while no other enemy has a stack of Investigation, target enemy gains a stack of Investigation (max 5) for 2 turns.
It clearly states that it applies Investigation if no OTHER enemy has Investigation. However even if you target the enemy with investigation that doesnt inflict them with any additional stacks of investigation. In game it works as if there is no enemy with investigation, not other enemy.
