Payment channel update is (indefinitely) delayed before sending to chain and it’s hard to work with the nonce requirement. The simplest way to handle it is to skip the nonce check for payment channel txn, and use the equivalent nonce inside the channel to prevent double spending attack. We need to make sure there is no other problem (especially security one) caused by this.
(Another way is receiver create a txn and put his own nonce, but it makes the data structure more complicated as there are two signatures in a single txn, and also sender’s balance will decrease without a nonce update. I don’t consider it to be an optimal solution)