OS 10.14.4 The “Paid” mark disappeared. Can I activate the “Paid “ gray stamp mark on invoice page instead of print preview?

Yes you can, Thomas Chan.

We can check you invoice template to see if the Paid watermark on paid invoices is unchecked. 

Let me show you how: 

1. From the Lists menu, select Templates. 
2. Double click on the invoice template you're using. 
3. Then, mark a check on the Print Status Stamp box. 
4. Once done, click OK. 

Let's create and pay an invoice to check if the gray PAID stamp is already showing. 

You can check out this article for more information about customizing your sales forms. 

Let me know if you need more help. Have a great day!