I have just deleted a bunch of pdf files that I previewed in pane, in window, had thumbnails on, flipped through a few pages in preview... and I am not able to reproduce the issue. Do you have any additional information on this?
Are these files on network or in other special place?
Did you enable these (and fully restarted OC from system tray)?
The only other thing is to ensure that
"UseNewPdfViewer": "True",
in OneCommanderV3.json
This cannot be changed in settings anymore but might be still "False" if you started using it a long time ago (when it was making this exact same problem).
Otherwise I will need more information. I unfortunately have not had MS Office in a decade to test with it