I’d have a look at this thread, and see about making the conversion with Calibre instead of Kindle Previewer. The Mobi file the latter creates isn’t very good for just reading with, it was meant for uploading to KDP back when Mobi is what they accepted.
I don’t know if that will solve your thumbnail problem, but it might, and it should improve the overall formatting too. I’d also try Calibre’s AZW3 format too, only very old Kindles will be unable to read that, and it is overall a better format to use, as Mobi has been deprecated for a long time now, and Calibre’s converter probably no longer receives updates to it.
But if you really do need .mobi, check the “MOBI output” tab in the conversion dialogue, and delete the “[PDOC]” tag. As I recall having the book tagged as a personal document could cause the thumbnail to not appear, on older devices.