I assume you're not entering an income account - your "Sales" account - on your bills directly, or you'd know why they're showing up on the P&L using the account.

Bills will use income accounts in cases where you use items / are purchasing items that have only one account, an income account.

To avoid this, edit the items you're purchasing on your bills and enable the purchase option.  It'll have a different title depending on the type of item, and your QB version, but it'll always appear in the location seen here:

When you check the box, then you'll see options for values to use when you purchase the item:

When you add the Expense account and then save the item, you'll get a prompt like this:

Answer Yes, and then all of your purchases that you don't want to use your sales account will be updated to use the new purchase/expense account.