I've bought several aftermarket covers for my various readers over the years. I like having options, and the reader manufacturers don't seem to put much effort into covers.
Usually this has been a good experience, but I just got a cover for my Clara HD that drained the battery with astounding speed.
My guess is that the magnet intended to trigger the sleep function is the problem. Those things have to be positioned perfectly. A bit to one side, or a cover that doesn't close all the way, and your battery goes berserk. Is the reader constantly cycling between being on and then off, and then on again?
It's too bad, because it's a very handsome cover. I think I will try to flatten it by stacking several heavy antique paper books on it for a while. (After removing the ereader, of course.) Maybe that will help.