I'll provide information and steps on printing the Vat number on the invoices.
It could be the Customer Vat number option on the Content section of your template was not checked. That's the reason the VAT number is not printing on your tax invoices. That said, I suggest checking the template you're using and making sure that the said option has a checkmark.
Here's how:
Click the Gear icon at the upper right corner.
Choose Custom form styles.
Select the template you're using when creating invoices.
Click the Edit link beside the drop-down arrow.
Choose the Content section. Then, click the header of the invoice.
Go to the Display section. Then, checkmark the Custom VAT number.
Hit Done.
Once done, you can now try to print your invoices to see the results.
If the issue persists, I suggest logging into your QuickBooks account through a private browser. This will help us determine if the problem is related to the browser's settings. And, private browsing doesn't store local files or cache.
Use these keyboard shortcuts based on the browser you're using.