Audio for courses not working offline

For the last year or so, I’ve been using Memrise for several Japanese courses. My main use case is learning in the London tube, which has really bad connectivity, so I upgraded to Pro to be able to download courses for offline use. I was very happy with it until, about a month ago, I replaced my phone for a new one of the same make and model (Google Pixel), and so had to reinstall the app and re-download all courses.

Since that happened, I have been unable to use the app in offline mode for anything involving sound: the sounds just won’t play. The do play when I’m online, though!

If I was to venture an hypothesis, I would say it looks like the app doesn’t download the media when it downloads the course, and then lazily downloads individual media files as it needs them, but I of course I don’t know what I’m talking about.

Help? This has made Memrise pretty much useless for me.