I was working on a "word tile" lesson in portrait orientation. For a moment my phone moved to the landscape orientation. When I shifted back to portrait orientation, the lesson had changed to a "translate this sentence" lesson!
It's a feature available with the app. If you prefer typing instead of choosing the words (which seems a better option for learning though tedious when done on a smaller screen), you can make use of this feature.
Regrettably, the "feature" is not reversible. Once it changes from tiles to words, you cannot restore it to tiles. I tried doing portrait -> landscape -> portrait -> landscape -> portrait to go from tiles to words to tiles.