Appears to be new in 6.3.
To reproduce:

• Create a dataset
• Upload 1 image file that's below the thumbnail generation cutoff
• Confirm that you are seeing the thumbnail on the dataset page
• Go to the collection page, confirm that the thumbnail is showing there as well, for this new draft dataset (i.e., this image file has become the designated thumbnail for the dataset)

Now publish the dataset.
In the process, this image file will be marked as though the thumbnail generation failed (dvobject.previewimagefail = true) in the db; making the default icon appear on the dataset page instead of the thumbnails. However, the DataFile will stay marked as the designated thumbnail for the dataset (via dataset.thumbnailfile_id), and that in turn will mess up the rendering of the collection page, like this:

(Note: it may be somewhat intermittent - I've seen the publishing succeed a couple of times without triggering this issue; just keep adding files and publishing until it happens)

If you go to Edit Dataset -> Thumbnails, the thumbnail will still show there perfectly intact. Un-assigning the file there and choosing the default icon instead fixes the collection page. Alternatively, setting previewimagefail = false in the db (via /api/admin/clearThumbnailFailureFlag/{id} or directly) fixes the proper thumbnail display on both pages.