Why are invoices not mapping to their respective income accounts in QBO?

if you configure the Income Account at the product/service level in QBO, it should assign the desired income account.

To do this:

Select the Gear icon on the Toolbar.
Under Lists, select Products and Services.
On the Action column of the Product/Service you want to edit, select Edit.
From the Income Account drop-down, select the new account.
(Important: Make sure the inventory item's account Detail Type in the Chart of Accounts is Sales of Product Income or else the Income Account choice will not show up.)
If you want all previous use of the item to be changed, mark the box Also update this account in historical transactions. (Note: This box only becomes available after the account is changed.)
Select Save and close.

By default, we sync to the Income Account configured in your QBO page in Recurly: https://yoursubdomain.recurly.com/integrations/quickbooks/edit

Once you have updated your products/services income account in QBO, we will sync to that selected income account.