i have same, placed 3 elephats and some another pets 2 times in animal pen t1 and put plant fiber, and 2 times when my pets should have grown and i came to take it all (not only elephants) pets i put here are disappear. the event log are empty both times
or this is a feature and animal pen t1 dont for elephants? but why are another pets disappear? and other stuffs in the game, for which improved workbenches are necessary, you just can’t start doing in t1