Tera Rule Issue

I've been trying to figure this out. I understand that Tera rule will prevent damage done to the Tera when on the bench. I believe, from what I could find, that if you have a duplicate Tera ex, one on bench and one in active, then the rule only is supposed to apply to the bench ex. However, a couple days ago, I was playing against a Tsareena ex and was unable to attack with damage to the Active Tsareena ex once a duplicate was placed on the bench. I then realized that it wasn't the same for my Charizard ex, Arcanine ex, or Garchomp ex I have played against or used. That Tsareena ex could not be damaged. It was registering a 0 every round I tried to attack. I triple checked that no other card effect was played. Is Tsareena ex bugged, did I encounter a cheater, or am I not understanding the Tera rule?