Just had a look at this and is seems the text in SVGs are not displayed in kepubs. The first file in a kepub is treated differently to the other files. But, I copied to a second file, and the same thing happened. I don't think I've used text in SVGs before. But, they work fine for images.
For the thumbnail, are you sure it is showing the first page? I would expect it to be the default cover Kobo uses which is just the title and format with two rectangles around them. The reason for this is that for kepubs, Kobo needs an image to be marked as the cover in the OPF. It extracts this and sizes as needed. As there are no images in the book, the default is used. For epubs, the first page is rendered, and that is used as the cover.
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