I believe there was an update in Oct 2020 for the "Banking" section in QuickBooks Online.

Now when I match a transaction to an existing bill in foreign currency ("Bill Payment"), the exchange rate will be 1. For example, I created a bill with amount JPY 75000, which equals to USD 712.35 according to the QB rate. After I paid the bill, the transaction in "Banking" was correctly linked to this bill. But after "Match", the exchange rate it used was 1, which means the amount in JPY and USD were both 75000 in QB.

It used to apply the exchange rate automatically. Now I need to go to the "Chart of Accounts" and select the transactions one by one to modify the exchange rate. Please fix it or move back to the old version for now.