Product: At least AuraHD and H2O
Firmware: It's been here for some time. Now I'm in 3.11
Plugins: None
File types: At least in sideloaded kepubs
Sideloaded or from Kobobooks: Sideloaded
On external SD: No
Steps to reproduce: No know trigger
Current result:
This is a small issue but I've seen it for a long time in my old HD and it also happens in my new H2O.
Sometimes the cover in the Sleeping / Powered off screen is corrupted somehow. It's substituted by a blank rectangle.
If you don't have the setting about showing covers in sleeping / power off screen, you can also detect that this issue has happened because instead of a simple "Sleeping / Powered off" text, you also get the blank rectangle (I insist, even if you DON'T have the show cover option enabled).
Workaround: A simple power off / on cycle restores the normal behaviour.