Why doesn't Appsync subscription work for object input parameter?

I am using AWS appsync for graphql communication. I have below schema for subscription:

type Subscription {
    requestTransaction(id: ID!): Transaction!
        @aws_subscribe(mutations: ["transactionResponse"])
}

it works fine. The subscription receives events when the transactionResponse mutation returns an evnet {id: '123'}. However, if I add a object type to the subscription requestTransaction like below code:

type Subscription {
    requestTransaction(transaction: TransactionInput!): Transaction!
        @aws_subscribe(mutations: ["transactionResponse"])
}

input TransactionInput {
    id: ID!
}

the subscription doesn't receive any events even if mutation response {transaction: {id: '123'}} event.

I wonder why Appsync doesn't support object input parameter?