You could be hitting pdfreport server app page timeout, defaults to 180 seconds.
Splunk will wait for dashboard's print ready event or timeout event to occur, which ever happens first, to actually generate PDF. In your case it could be that it takes more than 180s to generate your 50 pager thereby resulting in empty modules in pdfs.
Try to update this firefox_timeout config in $SPLUNK_HOME/etc/apps/pdfserver/local/pdf_server.conf file to a larger number, equivalent to time in seconds it will take to load the complete page.
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