[I tried responding to you by replying to the email notification of your message, but it doesn’t seem to have posted here, so my response may be duplicated.]
I am using a iPad Pro 9.7” under iPadOS 13.1.3
I even tried clearing the cached data for SitePoint (before trying again). One thing I did notice is that I had difficulty logging out - I got a few messages along the lines that the problem had been logged and reported - but I managed eventually and logged-in again just to be sure that I was starting afresh.
It’s frustrating, because this is a feature I’ve been waiting for.
Just to be sure we’re looking at the same thing, can you give me a title that works for you so that I can try that. (I know there are various legacy rights to download some books which could involved.)
Note: I have just managed to get this to work as intended on a (desktop) Mac under macOS 10.14.6; but - obviously - it’s not of the same utility as being able to read offline on an iPad!