I haven't found any ongoing investigations about not showing all items when printing invoices in QuickBooks Online. To narrow down the cause of this unexpected behavior, we can start by performing the basic troubleshooting steps to resolve the problem.
First, open QuickBooks using a private window and try printing an invoice from there. Sometimes, the regular browser tends to hold onto information, and over time it could cause problems while working with QuickBooks.
To save you time, use either of these keyboard shortcuts: