Bugs happen. You are supposed to report them using the "Report Problem" tool in settings for the app.
I was able to get the same bug to occur on my phone after a couple of tries.
It is sort of an interesting question to wonder about how such a thing could occur.
I was playing as white against the computer on the Android app.
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. d4 exd4 5. Nxd4 Bb4 6. Nxc6 Bxc3+ 7. bxc3 dxc6 8. Qxd8+ Kxd8 9. Bg5 Re8 10. Bd3 b5 11. O-O axb6????
The computer somehow captured an invisible piece on b6. Amazing! Does anyone else experience bugs with this software? How does this happen?