This is because the payments to Brex pulled from the bank feed will typically have the date that the payment left your bank account, whereas the payment to Brex pushed from the Brex dashboard will have the date that it reached Brex's bank account. Because of this date discrepancy, the payments will not auto-match. To rectify this, please follow these steps:
Step 1: On QuickBooks Online, go to Transactions > Banking and find the payment to Brex.
Step 2: Click on the payment, and click Find Match.
QuickBooks will suggest the payment to Brex that is a duplicate for the same amount. Each line will prompt you to match the transaction in your bank feed to the expense data, combining the two to reconcile.