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: Go to Transactions > Banking on QuickBooks Online and find the payment to Brex
Step 2: Click on the payment, and click on Find Match
QuickBooks will suggest the payment to Brex that is duplicated for the same amount so that you are able to match these.