Ecommerce purchase value is not decreased after a refund event in GA4

I'm migrating from UA to GA4 for ecommerce tracking.

I'm testing buy-and-refund scenario with GA: I sent a purchase event to GA4 via measurement protocal; After I ensured the purchase event is received (value showed in Ecommerce Purchases), I sent a refund event with the same transaction ID (through measurement protocal as well). I waited a day, but the value of purchase values in "GA4 Monetization -> Ecommerce Purchases" didn't decrease.

I'm sure the refund event is received (from Realtime report and Explore) by GA4. Is this a bug or expected behaviour? I didn't see it is described anywhere. Did I make any mistakes in event payload?

Here are my event payloads:

          {
            "name": "purchase",
            "params": {
                "items": [
                    {
                        "item_id": "L-6",
                        "item_name": "Product Report - 1775",
                        "item_category": "Report",
                        "quantity": 1,
                        "price": 60,
                        "currency": "CNY"
                    }
                ],
                "currency": "CNY",
                "transaction_id": "LP-6",
                "value": 60
            }

        {
            "name": "refund",
            "params": {
                "currency": "CNY",
                "transaction_id": "LP-6",
                "value": 60
            }
        }

When I was using UA, I could send negative revenue in transaction hit and negative quality in transaction_item hit in order to refund and decrease reveune. It seems this doesn't work in GA4? Should I should negative value in refund event? I didn't see any docs about it.