When I email an invoice to customer why is the invoice number not automatically entered in the subject line as it was previously?

I'll help you locate the invoice number in the new invoice interface in QuickBooks Online (QBO), damonsy.

The invoice number is visible on the Invoice page when generating an invoice, as displayed in the screenshot below:

However, when you click the Review and send, the invoice number is not visible on the Invoice Preview page. But once sent and received by your customer, the invoice number will be visible on the sent e-invoice.

Here's a sample of a sent invoice on the customer's end:

Additionally, you may modify your sales forms in QBO. This feature helps you manage what specific details to show on your sales transactions and personalize its appearance. Please check this article for more information: Customize sales forms in QuickBooks Online.

I'm always available if you have more questions about your e-invoices in QuickBooks. Just comment down below, and I'll make sure to provide accurate assistance. Have a good one.