I’m using the iOS version on iPad Pro. I follow the instructions on the website for putting it on my home screen as an app, and have used it for several hours. I don’t have this issue on the Windows version. It can’t be a restriction on mobile devices though, because their features page clearly advertises that it can be used offline immediately before and after paragraphs detailing the ability to use it on mobile devices as well as pc and Mac. I doubt they would put something so blatently misleading like that if it were the case that the offline feature only works on pc and Mac, especially since they don’t list that as a restriction.