Hi Niyas. Specifically for Odoo I'm running  18.0-20241008 (Community Edition) For wkhtmltopdf have now tried the following packages with no change. 

wkhtmltopdf_0.12.5-1_amd64.deb

wkhtmltopdf_0.12.6-2+b1_amd64.deb

wkhtmltopdf_0.12.6-2+b2_amd64.deb

Header is still missing. I restarted the Odoo service after each was installed, then tried new and duplicate invoices.

Also I'm finding that the PDF displayed in the right pane on the right side of the invoice page, and in the customer portal is an older version of the invoice with a different dollar amount. Almost like its cached on the Odoo side? I tried an incognito tab to isolate anything cached in browser. Again no change.

Update: I finally found where the attachment is on the invoice page to create an updated PDF. Running 12.6-2 still no header.

Removing the .deb for 12.6-2+b2 along with an autoremove for dependencies.

Hi,
It is mostly due to the version issue with wkhtmltopdf package, can you try installing 0.12.5 and see if it makes any difference.

Thanks