This seems to be a problem with the program used for HTML-to-PDF conversion (by default wkhtmltopdf I believe) so you will have to take it up with them as it isn't properly a Pandoc problem.
Development on wkhtmltopdf has AFAIK stopped, so I would advise you to first try another HTML-to-PDF engine first, e.g. weasyprint, and if you still have problems take it up via their channels. It is possible that you will be better off first producing an HTML document with Pandoc then invoke the HTML-to-PDF engine separately, passing it your HTML, your CSS and whatever other options or configuration which may be needed.
Please refer to