I took a closer look at my Google Drive setup. The file on my Kobo ereaders show up in .kobo/google_drive and not in kobocloud/Library when downloaded. The front cover thumbnails are all over the place depending on how large the image in the ebook is with no stretch to fit but they are there. This is with 4.38.23038 on a Sage and 4.39.23027 on a Libra Colour. The thumbnails match what I see with the same book on Dropbox. This suggests that you are using a 3rd party tool to sync with Google Drive so discuss the issue in the thread for that tool. Probably the
Support for more cloud services: Google Drive, Dropbox, etc. or the author's
Github site.