Yep, just the ones for the non-PDOC books added via calibre's send-to-device feature. calibre generates the thumbnails for those, and needs a more recent version to handle the increased resolution on the Voyage-and-probably-PW3.
It is literally just a problem of telling calibre to automatically make a
bigger image. You could resize the images yourself, but that is way too much work!
So you end up with a tiny thumbnail that doesn't look quite as good
and we can't have that.
Books sideloaded via your File Browser won't have a cover thumbnail unless they are a PDOC, and books downloaded directly from Amazon, both get their thumbnails via the device's internal processes, and the covers will be fine.