Yeah, clickbank don't delet commission. Sometimes, a buyer ask for a refund, and you will lose the commission. It also happen with chargebacks. It's like that, there is nothing you can do, take it easy and promote more. Also, be sure to choose good products.
Well I also lost a commission in the past from a clickbank sale. Refunds happens, and it is correct. I'm sure they didn't steal your commission.
Why will CB delete commissions? It may be the refunds as discussed. See in your account, everything will be furnished.