They did exactly what they are meant to do: logically closing the fence loop and thus creating a valid space in-between to be recognized by the program. They do NOT prevent the animals from escaping. At all! And they never were meant to do so. You’ll need additional measures to close the area “physically”: Rocks, building walls, steep slopes.
Animals did escape through them, if for example the rocks placed had too large spaces between them. This can be tested via the “accessible space heat-map”, which will color every space blue, an animal can walk on and also highlight “escape vectors”. But this is calculated in relation to the animal’s size and while mostly adults were tested on habitat creation (where everything worked perfectly fine), babies could weasel through way smaller gaps and just escape through them.
However, there was also a bug in the beta, where animals did escape through perfectly valid enclosure walls. This had nothing to do with null-fences, though, but also happened with other barrier types.
A “trick” for the use of null fences:
Make sure, the null fence is definitely behind the artificial wall (whatever you chose to use). If you cut corners and there is a small spot reachable for the animal, it will go there and technically be considered escaped, even if it can not leave the enclosure any further and would be well contained, if this were a real habitat.
Hence vets will come, box it and put it “back” into the habitat.