I cant see anything that stands out in the packages spec file as interfering with thumbnails.
However, I checked on KDE’s bug portal and found a few issues reported there relating to thumbnails and Oxygen theme:
https://bugs.kde.org/show_bug.cgi?id=504126
https://bugs.kde.org/show_bug.cgi?id=504913
and one semi-related but with no mention of Oxygen with this one:
https://bugs.kde.org/show_bug.cgi?id=503799
Maybe some of those symptoms match with what you experienced with the plasma6-oxygen package installed?