Weird. Is that steel mesh along one barrier edge? Did you make sure to make it unclimbable? (I always forget to do this lol).
It's a little hard to tell from the screenshot, any chance they could climb one of those trees and jump over the wall? Haven't played with tigers yet so I don't know what there jumping ability is (or if they even can jump from tree in this game)
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Are those pieces made of wood? What about that canopy? Even if they are on the other side of the wall, if they are climbable pieces they have priority over any other object for climbing animals: they can use them to climb and therefore clip through the wall. I had a similar problem with tigers too.
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That seems to be a climbable log. If one of its edges reaches the wall, even if slightly, it allows the tiger to go through it.