Doesn't save work when it crashes – Mathdoku+ Sudoku Style Math & Logic Puzzle Game Review

The app crashes not infrequently. Or even if it's just running in the background and closes. When you go back in, your work is gone. Some of the puzzles take quite a long time, so this is a problem. Also, some of the puzzles are easy and some are impossibly hard, with no apparent logic or order.
Review by Jillalalala on Mathdoku+ Sudoku Style Math & Logic Puzzle Game.