Well. The point that I decided to reinstall MS Mahjong several days ago. Since uninstall I cannot get it from MS Store with Code: 0x803F8001 (last checked a minute ago). So maybe it works for you - but for me not because Store returns the error.
Well, hopefully it means they are in the process of fixing it, then. I developed this problem on 7/7 with Minesweeper and finally was able to re-install it just a few minutes before I posted. I'll keep an eye on things, but I hope this means they're rolling out a fix (which probably will take some time to affect everyone, I imagine?)