After Bills sync to QuickBooks Online you will receive an email titled Viewpost Auto-Sync Results. The Email will tell you how many bills and credit memos were and were not synced.
After receiving the email go to QuickBooks Online and run a report for the Unclassified Sync Expense account. All bills and credit memos that have synced are posted to this account and listed on the report. Click on the bill and update the bill to the correct account or items. This will remove the bill from Unclassified Sync Expense account.