Cover thumbnails on my Oasis are no longer being replaced by the generic cover image. I don't know when this started. I saw it
mentioned on Reddit and decided to try it for myself today.
The person on Reddit who pointed it out assumed that the change was in the latest Kindle firmware. I doubt that.
It seems more likely to me that Amazon's servers have changed to no longer deliver a missing cover image file. This assumption can be tested by seeing if the problem has also gone away for those not running the latest firmware. (I don't have a device with appropriate firmware to check at the moment.)
Good news for now. Of course this may have been unintentional and Amazon could always decide to change it back.
Added: After more testing I see that I was wrong in my assumption. Amazon's servers still deliver the missing cover image file for unknown books. So it does appear that my Kindle is actually now being less aggressive about trying to replace the cover.