This is happening on iOS (I haven’t tried it on Android yet).
When I try to hold down on an image to save it, the image just goes full screen and the UI disappears. I don’t know how to go back, so I end up killing the app and starting over. There’s a download function, but it also just makes the image go full screen and the UI disappears again. Clicking the button download does not download the image (at least it doesn’t show up in recents).
On Android, there’s a hardware and OS back button. With Apple, you have to swipe, but swiping doesn’t work in this situation.