User creates an invoice for a project and then that project immediately disappears from the Projects screen.
This can happen if creating the invoice makes the total amount billed on the project equal to the contract amount while the Settings > Billing & Invoices > General option Mark projects completed when billed 100% is checked. As a result, CORE changes the status of the project to Completed and it is no longer visible in the Projects screen. This is because, by default, CORE only shows the projects with Active status in the Projects screen. To see projects with a status other than active, you need to use a filter on Project Status. Check out this FAQ for details: Why can't I find my projects in the list.