When exporting to PDF a multiple tab file originally developed in Visio, and edited in Draw.io v 12.9.13, the resulting PDF includes all the pages, but all displayed in portrait, although the tabs were all done in landscape format
From Chrome v 80.0.3987.163 (Official Build) (64-bit)
File > Export as > PDF > All Pages plus "Include a copy of my diagram > Export.
I can send the original and pdf files, but would prefer not to post here, if possible.