I figured I just put this here since it's not too big of a topic to make its own thread and it's related to send to kindle.
Currently, the display cover for documents when the Kindle is asleep is not working. It just shows the generic Kindle wallpapers. From this
reddit thread, it seems like people started noticing it starting yesterday. One user reported that it happened to some people a few weeks ago. Amazon might be slowly implementing this across all of their Kindles since it's happening gradually, or we're just the few unlucky ones chosen as test subjects. Or some intern is unhappy with Amazon and decided to mess about. I hope it's not permanent.
This is kinda annoying since I just recently made blank books for custom wallpapers.
Anyway, it's only happening if your Kindle is online, so if you like the book covers and your Kindle has been on airplane mode for a while, better not take it out of it.