Imagine that opponent have deck have full of "stop opp bonus" cards, like nightmares and you have 2 Leader card in your hand.
Leader bonus = calcel Leader
lets imagine more, your leaders are Ambre (courage 3) and Hugo (attack +6), both in your hand.
You play Ambre with the bonus "cancel Leader" and opponents plays whatever with the bonus "stop opp bonus"
In my thinking, it stops ambres bonus "cancel Leader", so Hugo during the attack is not cancelled, so it should give the attack of +6 to Ambre.
Question is : does it work like that ?
Leader are the only acception to the same card Bonus rule :( i tested the theory a long while back, hoping that i could power level Vansaars...
but, as for your scenario, you fail to realize that Hugo also has Cancel Leader, so, even though Ambre Bonus is canceled, Hugo bonus will still take affect. therefore still canceling Leader abilities.