This is a common misunderstanding. Don't worry, you didn't lose it. Channels are not static. The funds move between them as you route funds. When a channel closes you receive how much sats you still had on that channel at the moment of closing. The "missing" sats are on your other channels. You can find the routings in your forwarding history.

You did not lose fund, it was probably forwarded to another node. But I agree that there's misunderstanding in the wording. "Cooperative" probably means your node is responsive... it doesn't mean you, as a user, needs to explicitly agree to channel closure... Otherwise, do let me know if there's such setting/option.

In terms of cheating, it's possible if your node is offline for a long long long time. Channels connected to you can broadcast an old state where they had more funds on their side, and force close... which takes a couple of days... If your node becomes online before then, they risk on losing all their funds.