Unfortunately this looks to be a known limitation of the export to PDF functionality in Qlik Sense currently. On the release notes for Qlik Sense it specifies that "Custom visualization extensions cannot be exported".
Do keep an eye on future releases notes to see when/if the developers are able to integrate the ability to export custom visualizations.
Just for clarity, example extensions provided by Qlik are not part of the core product. These are examples that were built by internal Qlik staff and developers for the purpose of demonstration and example. Therefore they are not in the code track for the core product and do not support the export feature.
Hope this is clear and you continue to enjoy using the product.
Let us know how else we can help.