This is a very frustrating bug that I am experiencing all of the time. I want to listen to some lessons on the way or on the train where I do not have internet, so I download them before. I wait until the download progress is 100% and then disappears, so lesson should be downloaded. But then on the train, try to play the lesson, it doesn't work. When I start the dowloaded lesson still having wifi and the leave the house, it plays for some minutes and then stops, so it seems like it just streamed the lesson as usual and doesn't use the downloaded file. Trying to start the lesson on the way doesn't work again, as if the lesson was never downloaded.
This is very annoying, since it is a huge waste of time when I cannot study on the train since the app didn't work.
I am not a paying user any longer anyways, but just wanted to let you know that you should really get this fixed!